MCAT Chemistry Flashcards

Question

Sodium hypochlorite (NaClO) is used as a disinfectant and has also been shown to treat chronic skin inflammation. What is the oxidation number of chlorine in NaClO? A) -1 B) 0 C) +1 D) +2

Answer 1

C In NaClO (sodium hypochlorite), sodium carries its typical +1 charge, oxygen carries its typical -2 charge. This means that the chlorine atom must carry a +1 charge in order to balance the overall charge of zero.

Answer 2

D Acids have pH values of less than 7, with the stronger the acid the lower the pH. Bases have pH values of greater than 7, with the stronger the base the higher the pH. Typically: Strong acids have a pH of 0 or less, hydrochloric acid pH = 0. Weak acids between pH 3-6, acetic acid pH = 4. Neutral solutions have pH = 7, water pH = 7. Weak bases have a pH 8-10, ammonia solution pH = 10. Strong bases have a pH of 14 or more, sodium hydroxide solution pH = 14.

Answer 3

A Zero order reactions proceed at a constant rate. The time it takes to go from one-half to one-quarter of the original amount will be only half the time needed to go from all of the reactants to one-half of them. (Fun Fact: A first-order reaction would take 5 minutes.)

Answer 4

C Since K > 0, the reaction is spontaneous, and ΔG < 0; ΔG = -RTln(k) Although ΔH and ΔS combine to determine ΔG, we lack information to determine the sign of either of them.

Answer 5

B Cadmium in it’s basic state has Configuration [Kr] 5s^24d^10. Positively charged cadmium (II) has lost 2 electrons. It will lose its two 5s-electrons first, making B the most reasonable answer. Answer A is illogical, as it has a higher shell number (5) for the d-electrons than the s-electrons (4). C is the correct configuration for neutral cadmium. Finally, D is incorrect because the d-electrons were removed rather than the s-electrons.

Answer 6

D Rearrange Faraday’s Law: moles = I x t/n x F. Note that we have to convert min to s, and that 3 electrons are transferred in the redox reaction.

Answer 7

C The rate of consumption of the reactants will be the product of the rate of formation of the product multiplied by the ratio of the (co-efficient of the reactant) / (co-efficient of the product). Rate N2(g) = 1.50 x 1/2 = 0.75 mole.L-1.s-1 and H2(g) 1.50 x 3/2 = 2.25 mole.L-1.s-1.

Answer 8

B A catalyst lowers the activation energy of a reaction, causing an increase in the reaction rate.

Answer 9

B If the products have more stable bonds, that means they have less energy, and that energy must have been released, hence the change of enthalpy is negative. If the products are in a more orderly arrangement than the reactants, then the entropy decreased; thus the change in entropy is also negative, and the only answer left is B.

Answer 10

C Carbonate is a weak base and reacts with the H+ from the strong acid, causing the barium salt to dissociate. CO32- reacts with H+ to form HCO3–, causing the BaCO3 to dissociate more than it would in neutral water.

Answer 11

B The equilibrium of a reaction can be changed by several factors. Adding or subtracting heat would shift the equilibrium based on the enthalpy change of the reaction. Increasing reactant concentrations would shift the equilibrium in the direction of the product and the opposite would occur if reactant concentrations were decreased. Changing the volume of a reactant would affect any reaction with gaseous reactants or products. While adding or removing a catalyst would change the reaction rate, it would not change where the equilibrium lies.

Answer 12

B Note solubility, not solubility product, is hydroxide-dependent. D is wrong because [H+] and [OH-] are related.

Answer 13

C This solution is a buffer, for which: [H+] = Ka [HFor]/[For–] = Ka pH = -log (1.8 x 10^-4) = 3.7

Answer 14

B Alpha decay results in the loss of two protons, while ordinary beta decay results in the gain of one proton. Since isotopes of the same element have the same number of protons, the number of protons lost by alpha decay must equal the number gained by beta decay. Therefore, twice as many beta decays as alpha decays must occur, so the ratio of alpha to beta decays is 1:2.

Answer 15

B While in the area between the plates, the negatively charged electron is attracted to the positive plate and so it would bend towards the positive plate, or “upwards” in the diagram. But after leaving the plates, there is no more net force acting on the electron. Therefore, the electron would continue in motion in a straight line according to Newton’s first law (inertia).

Answer 16

D Note that [Cl^-] = 2x.

Answer 17

C The correct answer is C. The Arrhenius equation is given by k = Ae^-E/RT, where A is the pre-exponential factor, R is the gas constant, T is the temperature, and E is the activation energy. Thus, if the temperature were increased by a factor of 4, then the exponent would decrease by a factor of 4, since T is in the denominator. However, since it is e-x, the actual value won’t change much. For example, exp(-1) = 0.36, but exp(-1/4) = 0.77. Thus, although the value of k will increase, it will not increase by a factor of 4, only slightly.

Answer 18

C “The sublimation of carbon dioxide occurs quickly at room temperature” means that it is spontaneous and so ΔG must be negative (by definition). Sublimation means: Solid CO2 + heat -> vapor Entropy (randomness) is clearly increasing (thus positive ΔS) because we are moving from a structured, ordered solid to randomly moving gas particles. Heat is required so it is endothermic meaning ΔH is positive. The question is asking about the reverse reaction so all 3 signs are reversed: ΔG is now positive; ΔH is now negative; ΔS is now negative. Going deeper: Notice that a negative ΔS multiplied by a –T (see the Gibbs free energy equation) creates a positive term which overshadows the effect of the negative ΔH and thus ΔG is still positive. Also, please keep in mind that sometimes the real MCAT will provide the Gibbs free energy equation but sometimes they won’t.

Answer 19

C In this question, we use the ideal gas law, PV = nRT. First of all, we calculate the number of mols in the volume of 11.2 L. n = PV/RT, or n = (1*11.2)/(0.082*298). This gives us roughly n= 0.5 mol. You can bypass this by remembering that there are 22.4 L in a gaseous mol at STP. Thus, there is 0.5 mol of a substance that we know weighs 13g. The molar mass must be 26 g/mol then. With the knowledge that C = 12g/mol and H = 1g/mol (standard knowledge), we find that the hydrocarbon should be C2H2.

Answer 20

C A catalyst speeds up a chemical reaction by lowering the activation energy of a reaction. The catalyst is never consumed, nor does it alter the reactants or products. The total enthalpy of the reaction is also not changed from what it would have been without the catalyst. Only the rate at which the reaction occurs and the activation energy is affected. The only answer option that is correct is [C].

Answer 21

D Two gases having the same temperature have the same average molecular kinetic energy. Therefore, (1/2)m1v1^2 = (1/2)m2v2^2, which gives m2/m1 = (v1/v2)^2. Since v1/v2 = 2, we have m2/m1 = 2^2 = 4. Among the available choices, the only gases with a mass ratio close to 4 to 1 are krypton (83.8 g/mol) and neon (20.2 g/mol).

Answer 22

D The ground state for Cu is [Ar] 4s1 3d10. Transition metals always lose their valence s electrons before any d electrons, so the ground-state electron configuration for Cu+ [(I) means +1] is [Ar] 3d10. The only excited configuration for Cu+ must be [Ar] 3d9 4p1. An alternate strategy is to look at the number of electrons, as Cu+ has 28, and this helps eliminate A and C as they do not have 28 electrons.

Answer 23

D Only temperature will change the Ka, Kb, Kw, Keq, and Ksp. The Ksp will be constant and not change under the conditions provided in the answer choices.

Answer 24

D Ideal gases do indeed move faster when heated due to the increase in average kinetic energy. However, we assume that there are no intermolecular forces between molecules, therefore they are infinitely compressible in theory. We also assume that an ideal gas will not change into a liquid no matter the conditions.

Answer 25

C The meniscus is the curved upper surface of liquid. This is formed because of unequal cohesion and adhesion attractions. For example, in water the adhesive forces are greater than the cohesive forces, therefore the water is able to “walk up the sides” of the surface. In the case of water, a U-shaped or concave meniscus is formed. Mercury, for example has stronger cohesive attractions between the mercury molecules than adhesive attractions between the mercury and the glass surface, therefore producing a convex meniscus, which looks like an inverted U.

Answer 26

A | ``` M1V1 = M2V2 0.3)(V1) = (1.5)(2 V1 = 10 L ```

Answer 27

B As you move from left to right across a Period, metallic properties decrease (metals > metalloids > non-metals), atomic radius decreases (the atoms get smaller), electronegativity (the ability to attract electrons in a compound) increases, first ionization energy (the energy required to remove the outermost electron from a neutral atom) increases and electron affinity (the energy released when an electron is bonded to an atom) increases.

Answer 28

A The addition of a solute to a liquid always lowers the vapor pressure of the liquid.

Answer 29

A Carboxylic acids are considered to be among the strongest organic acids but in comparison with all types of acids, carboxylic acids are very weak. By far, the protonated form of answer choice A (acetic acid is the IUPAC name; ethanoic acid is the systematic way to name it) would be the weakest of that group (after all, we are comparing it with nitric acid, sulfuric acid and phosphoric acid). Logically, a strong acid would create a weak conjugate base, otherwise the acid would not give up the proton readily. Thus the reverse is true: a weak acid has a strong conjugate base.

Answer 30

C On the MCAT, numbers can be intimidating, but remember that if the answer choices have no numbers in them, we should think conceptually instead of empirically. Gibbs equation helps us determine if a reaction is spontaneous or non-spontaneous. It is given as: ΔG = ΔH – TΔS In this case, the enthalpy change ΔH is a negative number (exothermic). We know that the entropy change (ΔS) is also negative (the products are more ordered than the reactants) because the reactants are 2 moles of gas and the products are only 1 mole of gas (a synthesis of phosphorus pentachloride). Our equation now be read differently as: ΔG = (negative number) – temperature * (negative number) Temperature is in Kelvin and thus always a positive number, but we must consider two possibilities regarding temperature: (1) If this positive temperature is relatively low, the second part of the equation is a small positive number. Thus, if you add a small positive number to a negative number, it will probably still be negative (ΔG<0). (2) If the positive temperature is relatively high, the second part of the equation is a large positive number. If you add a large positive number to a negative number, it will probably become positive (ΔG>0). In both these cases, ambiguity is important because we don’t know the temperature or ΔS. The only answer that appropriately describes the case we have here is C.

Answer 31

C A pH above the isoelectric point means that the amino acid is in relatively basic conditions. A base will accept a proton (H+) from the amino acid, leaving the amino acid negatively charged.

Answer 32

C The second law of thermodynamics states that nature tends to favor a state of disorder or entropy. In other words, chemical reactions that are exergonic (release energy) and result in more products than reactants are favored. The only answer option that has more products formed in comparison to the reactants is answer option C.

Answer 33

B An ionic bond is one in which both parts of the bond are highly positive or negative so that they are attracted strongly to each other.

Answer 34

C From the balanced equation, we can see that 4 moles of Ag are produced for every 1 mole of O2. Therefore, if 1 mole of Ag is produced, then (1/4) mole of O2 is produced. The mass of (1/4) mole of O2 is (1/4)(2)(16 g/mol) = 8 grams.

Answer 35

D Boiling and Condensation Melting: solid to liquid and Freezing: liquid to solid occur at the Melting/Freezing Point. Boiling: liquid to gas and Condensation: gas to liquid occur at the Boiling/Condensation Point. Both evaporation: liquid to gas at the surface of the liquid and sublimation: solid to gas occur at any temperature.

Answer 36

D In each of II and II, the number of moles of gaseous products is less than that of gaseous reactants; thus “disorder” decreases.

Answer 37

A A calcium atom normally contains 20 electrons, so if it acquires a charge of +2, it must have lost 2 electrons, leaving just 18. Only choice A has 18 electrons.

Answer 38

C A weak acid will not dissociate much, meaning that A-/HA will be very low, and log(A-/HA) will be negative. Thus, a weak acid with a pKa of 10 will likely have a pH around 8 or 9.

Answer 39

B Because the electron is moving into the n = 1 shell, the only subshell available is the 1s subshell, which eliminates choices (C) and (D). There will be some energy change, however, as the electron must lose energy to return to the minimum-energy ground state. That will require emitting radiation in the form of a photon.

Answer 40

D STP is Standard Temperature and Pressure or 0oC and 1 atm pressure. At STP 1 mole of an ideal gas occupies 22.4 L. A good approximation of the molecular mass of a gas is obtained by multiplying its density in g.L-1 by 22.4 L to attain the mass of 1 mole of gas. Hence: 3.17 x 22.4 = 71.0 24.5 L is the molar volume of an ideal gas under LTP or Laboratory Temperature and Pressure, a balmy 25oC and 1 atm pressure.

Answer 41

D During a titration procedure, a buret is lowered into a clean Erlenmeyer flask. The initial volume of titrant is recorded and a stream of titrant is delivered while constantly mixing the sample solution. Once the pH indicator changes colors we have reached the endpoint. The final volume is recorded and used in conjunction with the initial volume to find the volume of titrant used, and along with the concentration of the titrant to find how many moles of titrant were used. Finally, we can use stoichiometry of the titration reaction to calculate the number of moles of reactant in our solution. The pH of the titrant is not needed.

Answer 42

C Charles’s Law states: V T Dalton’s law of Partial Pressures states: Ptotal = P1 + P2 + P3 + … Boyle’s gas law states: P 1/V The Ideal gas law states: PV = nRT

Answer 43

B The rate at which a chemical reaction occurs is increased by increasing the temperature, adding a catalyst, increasing the concentration of a reactant in solution, increasing the partial pressure of a reactant gas and increasing the surface area of a reactant solid. Catalysts increase the rate of reaction by providing an alternative way for the reaction to continue with a lower activation energy, thus making the reaction easy to occur under any given conditions.

Answer 44

B The correct answer is B. The best substitute is one that is the closest to Na on the periodic table. Remember, families in the periodic table have similar chemical properties. Thus, anything in the second group is unacceptable, eliminating answers A, C, and D.

Answer 45

B For the pH to equal the pKa, log(A-/HA) must be zero, which only occurs at log(1), meaning A-/HA = 1. Thus, the amounts of acid and conjugate base would be equal. This means that roughly half of the acid has dissociated.

Answer 46

B A very low acid dissociation constant Ka means that the acid is weak and thus very little product is made. Since pKa is -logKa, the negative sign means they are always opposites. Thus low Ka means high pKa.

Answer 47

D t is not necessary to calculate anything to solve this problem but we will go through the steps. The dissociation of water (note carefully that the ratio of the products is 1:1; also keep in mind that this is the basis of the neutrality of pure water: acid units = base units): H2O + H2O <-> H3O+ + OH– The expression for the self-ionization of water or the water dissociation constant: Kw = [H3O+][OH–] = [H+][OH–] At room temperature, keeping in mind that the dissociation of the ions is 1:1, we get: 10-14 = [H+][OH–] = [H+]2 And so [H+] = 10-7 and then -log[H+] = 7 = pH Doing the identical math but using Kw = 10-13 yields a pH of 6.5 BUT because the ratio of the ions is still 1:1 (if not, you would not have done the calculation) thus it must be neutral.

Answer 48

C First of all, we can immediately disregard answers A, B, and E. Nearly all compounds containing sodium, potassium, or zinc are soluble. Thus, we are left with answers C and D. Of the two, we realize that sulfur does not bind with sulfate. Sulfur sulfate does not exist. We recognize as well that lead compounds are rarely soluble, but lead perchlorate is. Thus, answer C is the correct answer.

Answer 49

C The correct answer is C. Temperature change does increase the solvation ability of gas into liquid, but not significantly. However, we can easily recognize that a warm soda loses air much faster than a cold soda. Thus, it seems that answer B could be correct. However, in a sealed vessel, the factor that will increase the amount of air dissolved most immediately is the increase of pressure. The pressure is increased when another gas in forced in the system, like argon. The increased pressure will drive the equilibrium to force more air into the water.

Answer 50

D Since Calcium is ionic and has attached to two chlorides ions which are anions, D is the correct answer. Boron does not form ions very well. A and B only have one ionic bond each.

Answer 51

B A catalyst lowers the activation energy, Ea, of a reaction.

Answer 52

D The dissociation of HF is given by HF -> (H+) + (F-) Let [H+] = [F-] = x. Although the actual equilibrium concentration of HF is (2-x) M due to dissociation, we assume that x is very small and ignore the difference. That is, we take [HF] = 2 M at equilibrium. Since the equilibrium expression for this reaction is Ka = [H+][F-]/[HF], we have Ka = x^2/2, so x^2 = 2Ka = 2(6.8 x 10^-4) = 13.6 x 10^-4, so x = 3.7 x 10^-2.

Answer 53

C According to ideal gas assumptions, all gas molecules are very far away from other gas molecules and their volumes are negligible. Should two gas molecules collide, their collision will be elastic, meaning no kinetic energy is lost (the average kinetic energy can only be changed by heating/cooling the gas). While it is true pressure is caused by the collision of gas molecules against the walls of the container, this is not an assumption for an ideal gas, but rather a general fact of nature. We typically want our ideal gases at low pressures too.

Answer 54

B Use P1V1 = P2V2. Temperature is constant and can be eliminated from the equation. If the volume increases, we know the pressure must decrease. Doubling the volume should then halve the pressure.

Answer 55

D You must subtract the pressure of water vapor from the total pressure, and change torr to atm. 1 atm = 760 torr = 760 mm Hg.

Answer 56

D Use n = PV/RT. Remember R is a constant and that all the units must be converted into atm of pressure, liters of volume, and Kelvin from Celsius.

Answer 57

A At low densities the correction terms to P and V become very small. Both correction terms, nb and an^2/V^2, depend on the number of gas molecules in a given volume, which is small at low densities. PV is not constant for the real gas as it is for an ideal gas.

Answer 58

D Quaternary nitrogen atoms cannot react to form an amide because it already has four bonds.

Answer 59

B The pressure exerted by a gas does not depend on its molecular weight or the number of atoms in its molecules. Changes in T or V, however, will affect the pressure.

Answer 60

C From +2 through -2, m can have 5 values. Each m value can have 2 values of m(s) associated with it, for a total of 10 states (i.e., the 10 3d electrons).

Answer 61

D A diamagnetic ion will have no unpaired subshells in its atomic orbitals. A paramagnetic ion will have one unpaired electron in its atomic orbitals, leading it to be slightly more magnetic due to the unbalance. For example, Zn2+ will lose 2 electrons from the 4s2 while leaving the 3d10 orbital full, leaving no unpaired electrons. Copper will lose electrons from the 4s2 orbital to fill the 3d10 orbital, a more stable configuration and thus a diamagnetic compound.

Answer 62

D The electron in the neutral atom occupies an s orbital of higher n than the valence electrons of the positive ion. The former electron will stay, on average, farther from the nucleus than the inner electrons.

Answer 63

D By mixing a weak acid with NaOH, we convert some of the acetic acid to acetate. The resulting mixture of the acid and its conjugate base is a buffer, and [H+] will be within an offer of magnitude of Ka.

Answer 64

A In the primitive atmosphere, no O2 was present as most organisms used chemosynthesis to create organic compounds. The Oparin and Haldane hypothesis states that methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O) were the only gases present. It was after the Miller and Urey experiment that it was proven that these gases and lightning could have given rise to simple organic compounds that then later evolved into more complex forms of life.

Answer 65

B The azimuthal quantum number l cannot be higher than n - 1, ruling out A. The mt number, which describes the chemical's magnetic properties, can only be an integer value between -l and l. It cannot be equal to +/-1 if l = 0.; this would imply that an s-subshell has three orbitals (-1, 0, and 1) when we know it can only have one. This rules out C and D.

Answer 66

C For any value of n, there will be a maximum of 2n^2 electrons; that is, two per orbital. This can also be determined from the periodic table. There are only two elements (H and He) that have valence electrons in the n = 1 shell. Eight elements (Li to Ne) have valence electrons in the n = 2 shell. This is the only equation that matches this pattern.

Answer 67

B This formula describes the number of electrons in terms of the azimuthal quantum number l, which ranges from 0 to n - 1, with n being the principal quantum number. Subshell Azimuthal (l) # of electrons s 0 2 p 1 6 d 2 10 f 3 14

Answer 68

D The only answer choice without unpaired electrons in its ground state is helium. Recall from the chapter that a diamagnetic substance is identified by the lack of unpaired electrons in its shell. A substance without unpaired electrons, like helium, cannot be magnetized by an external magnetic field and is actually slightly repelled. Elements that come at the end of a block (Group IIA, the group containing Zn, and the noble gases, most notably) have only paired electrons.

Answer 69

A Recall that the superscript refers to the mass number of an atom, which is equal to the number of protons plus the number of neutrons present in an element. Sometimes a text will list the atomic number, Z, as a subscript under the mass number, A. According to the periodic table, carbon contains six protons; therefore, its atomic number is 6. Isotopes all have the same number of protons, but differ in the number of neutrons. Almost all atoms with Z > 1 have at least one neutron. Carbon is most likely to have a mass number of 12, for six protons and six neutrons, as in choice B. Choices C and D are possible isotopes that would have more neutrons than 12C. The 6C isotope is unlikely. It would mean that there are 6 protons and 0 neutrons. This would result in a highly unstable isotope.

Answer 70

C The limitations placed by the Heisenberg uncertainty principle are caused by limitations inherent in the measuring process: is a particle is moving, it has momentum, but trying to measure that momentum necessarily creates uncertainty in the position. Even if we had an exact definition of the meter, or perfect measuring devices, we still wouldn't be able to measure position and momentum simultaneously AND exactly.

Answer 71

B For the electron to gain energy, it must absorb energy from photons to jump up to a higher energy level. There is a bigger jump between n = 2 and n = 6 than there is between n = 3 and n = 4.

Answer 72

A The MCAT covers the topics in this chapter qualitatively more often than quantitatively. It is critical to be able to distinguish the fundamental principles that determine electron organization, which are usually known by the names of the scientists who discovered or postulated them. The Heisenberg uncertainty principle refers to the inability to know the momentum and position of a single electron simultaneously. The Bohr model was an early attempt to describe the behavior of a single electron in a hydrogen atom. The Rutherford model described a dense, positively charged nucleus. The element shown here, phosphorus, is often used to demonstrate Hund's rule because it contains a half-filled p subshell. Hund's rule explains that electrons fill empty orbitals first before doubling up electrons in the same orbital.

Answer 73

A The quickest way to solve this problem is to use the periodic table and find out how many protons are in Cs atoms; there are 55. Neutral Cs atoms would also have 55 electrons. A stable Cs cation will have a single positive charge because it has one unpaired s-electron. This translates to one fewer electron than the number of protons or 54 electrons.

Answer 74

B The easiest way to approach this problem is to set up a system of two algebraic equations, where H and D are the percentages of H (mass = 1 amu) and D (mass = 2 amu), respectively. Your setup should look like the following system: H + D = 1 (percent H + percent D = 100%) 1 H + 2 D = 1.008 (atomic weight calculation) Rearranging the first equation and substituting into the second yields (1 - D) +2D = 1.008, or D = 0.008. 0.008 is 0.8%, so there is 0.8% D.

Answer 75

A The terms in the answer choices refer to the magnetic spin of the two electrons. The quantum number ms represents this property as a measure of an electron's intrinsic spin. These electrons' spins are parallel, in that their spins are aligned in the same direction (ms = +1/2 for both species).

Answer 76

B The periodic table is organized into periods (rows) and groups (columns). Groups are particularly significant because they represent sets of elements with the same valence electron configuration, which in turn will dictate many of the chemical properties of those elements. Although choice A is true, the fact that both ions are positively charged does not explain the similarity in chemical properties; most metals produce positively charged ion. Choice C is not true because lithium and sodium are in the same group, not period. Finally, although lithium and sodium do have relatively low atomic weights, so do several other elements that do not share the same properties.

Answer 77

A As one moves from top to bottom in a group (column), extra electron shells accumulate, despite the fact that the valence configurations remain identical. These extra electron shells provide shielding between the positive nucleus and the outermost electrons, decreasing the electrostatic attraction and increasing atomic radius. Because carbon and silicon are in the same group, and silicon is farther down in the group, silicon will have a larger atomic radius because of its extra electron shell.

Answer 78

C Atomic radius is determined by multiple factors. Of the choices given, the number of valence electrons does have an impact on the atomic radius. As one moves across a period (row), protons and valence electrons are added, and the electrons are more strongly attracted to the central protons. This attraction tightens the atom, shrinking the atomic radius. The number of electron shells is also significant, as demonstrated by the trend when moving down a group (column). As more electron shells are added that separate the positively charged nucleus from the outermost electrons, the electrostatic forces are weakened, and the atomic radius increases. The number of neutrons is irrelevant because it does not impact these attractive forces.

Answer 79

C Ionization energy increases from left to right, so the first ionization energy of lithium is lower than that of beryllium. Second ionization energy is always larger than first ionization energy, so beryllium's second ionization energy should be the highest value. This is because removing an additional electron from Be+ requires one to overcome a significantly larger electrostatic force.

Answer 80

B Antimony (Sb) is on the right side of the periodic table, but not far right enough to be a nonmetal. It certainly does not lie far enough to the right to fall in Group VIIA (Group 17), which would classify it as a halogen. While sources have rarely classified antimony as metal, it is usually classified as a metalloid.

Answer 81

C Electronegativity describes how strong an attraction an element will have for electrons in a bond. A nucleus with a larger effective nuclear charge will have a higher electronegativity; Zeff increases toward the right side of a period. A stronger nuclear pull will also lead to increased first ionization energy, as the forces make it more difficult to remove an electron. The vertical arrow can be explained by the size of the atoms. As size decreases, the positive charge becomes more effective at attracting electrons in a chemical bond (higher electronegativity), and the energy required to remove an electron (ionization energy) increases.

Answer 82

C All four descriptions of metals are true, but the most significant property that contributes to the ability of metals to conduct electricity is the fact that they have valence electrons that can move freely. Malleability is the ability to shape a material with a hammer, which does not play a role in conducting electricity. The low electronegativity and high melting points of metals also do not play a major role in the conduction of electricity.

Answer 83

B This Group represents the alkaline earth metals, which form divalent cations, or ions with a +2 charge. All of the elements in Group IIA have two electrons in their outermost s subshell. Because loss of these two electrons would leave a full octet as the outermost shell, becoming a divalent cation is a stable configuration for all of the alkaline earth metals. Although some of these elements might be great conductors, they are not as effective as the alkali metals. Choice C is also incorrect because, although forming a divalent cation is a stable configuration for the alkaline earth metals, the second ionization energy is still always higher than the first. Finally, Choice D is incorrect because atomic radii increase when moving down a group of elements because the number of electron shells increases.

Answer 84

B Iron is a transition metal. Transition metals can often form more than one ion. Iron, for example can be Fe2+ or Fe3+. The transition metals, in these various oxidation states, can often form hydration complexes with water. Part of the significance of these complexes is that, when a transition metal can form a complex, its solubility within the related solvent will increase. The other ions given might dissolve readily in water, but because none of them are transition metals, they will not likely form complexes.

Answer 85

D This question is simple if one recalls that periods prefer to the rows in the periodic table, while groups or families refer to the columns. Within the same period, an additional valence electron is added with each step toward the right side of the table.

Answer 86

B This question requires knowledge of the trends of electronegativity within the periodic table. Electronegativity increases as one moves from left to right for the same reasons that effective nuclear charge increases. Electronegativity decreases as one moves down the periodic table because there are more electron shells separating the nucleus from the outermost electrons. In this question, chlorine is the furthest toward the top-right corner of the periodic table.

Answer 87

D The effective nuclear charge refers to the strength with which the protons in the nucleus can pull on electrons. This phenomenon helps to explain electron affinity, electronegativity, and ionization energy. In choice A, the nonionized chlorine atom, the nuclear charge is balanced by the surrounding electrons: 17 p+/17 e-. The chloride ion has a lower effective nuclear charge because there are more electrons than protons: 17 p+/18 e-. Next, elemental potassium, has the lowest effective nuclear charge because it contains additional inner shells that shield its valence electrons from the nucleus. Ionic potassium has a higher effective nuclear charge than any of the other options do because it has the same electron configuration as Cl- (and the same amount of shielding from inner shell electrons as neutral Cl) but contains two extra protons in its nucleus: 19 p+/18 e-.

Answer 88

D Ionic bonds are formed through unequal sharing of electrons. These bonds typically occur because the electron affinities of the two bonded atoms differ greatly. For example, the halogens have high electron affinities because adding a single electron to their valence shells would create full valence shells. In contrast, alkaline earth metals have very low electron affinities and are more likely to be electron donors because the loss of two electrons would leave them with full valence shells. Choice A states the opposite and is incorrect because the halogens have high electron affinity and the alkaline earth metals have low electron affinity. Choice B is incorrect because equal sharing of electrons is a classic description of covalent bonding, not ionic. Choice C is a true statement, but is not relevant to why ionic bonds form.

Answer 89

C When n = 3, l = 0, 1, or 2. The highest value for l in this case is 2, which corresponds to the d subshell. Although the 3d block appears to be part of the fourth period, it still has the principal quantum number n = 3. In general, the subshells within an energy shell increase in energy as follows: s < p < d < f (although there is no 3f subshell).

Answer 90

B Carbon monoxide, CO, has a triple bond between carbon and oxygen, with the carbon and oxygen retaining one lone pair. In polar covalent bonds, the difference in electronegativity between the bonded atoms is great enough to cause electrons to move disproportionately toward the more electronegative atom but not great enough to transfer electrons completely. This is the case for CO. Oxygen is significantly more electronegative than carbon, so electrons will be disproportionately carried on the oxygen, leaving the carbon atom with a slight positive charge.

Answer 91

B To answer this question, one must understand the contribution of resonance structures to average formal charge. In Choice B there are three possible resonance structures. Each of the three oxygen atoms carries a formal charge of -1 in two out of the three structures. This averages to approximately -2/3 charge on each oxygen atom, which is more negative than in the other answer choices. Both water and formaldehyde (A and D) have no formal charge on the oxygen. Ozone, Choice C, has a -1/2 on two of the three oxygens and a +1 charge on the central oxygen.

Answer 92

D The key to answering this question is to understand the types of intermolecular forces that exist in each of these molecules because larger intermolecular forces correspond to higher boiling points. Kr is a noble gas with a full octet, so the only intermolecular forces present are London dispersion forces, the weakest type of intermolecular forces. Acetone and isopropyl alcohol are both polar, so both have dipole-dipole interactions, which are stronger than dispersion forces. However, isopropyl alcohol can also form hydrogen bonds, increasing its boiling point. Finally, the strongest interactions are ionic bonds, which exist in potassium chloride.

Answer 93

C The central carbon in carbonate has no lone pairs. It has three resonance structures, each of which involves a double bond between carbon and one of the three oxygens. Having made four bonds, carbon has no further orbitals for bonding or to carry lone pairs. This makes carbonate's geometry trigonal planar. Alternatively, ClF3 also has three bonds; however, chloride still maintains two extra lone pairs. These lone pairs each inhabit one orbital, meaning that the central chloride must organize five items about itself: three bonds to fluorides and two lone pairs. The best configuration for maximizing the distance between all of these groups is trigonal bipyramidal. Choices A and B are true statements but do not account for the difference in geometry.

Answer 94

A The best way to approach this problem is to draw the structures of each of these molecules, then consider the electronegativity of each bond as it might contribute to an overall dipole moment. HCN is the correct answer because of the large differences in electronegativity aligned in a linear fashion. There is a strong dipole moment in the direction of nitrogen, without any other moments cancelling it out. Water has two dipole moments, one from each hydrogen pointing in the direction of the oxygen. The molecule is bent, and the dipole moments partially cancel out. There is a molecular dipole, but it is not as strong as in HCN. Sulfur dioxide has a similar bent configuration, and its dipole will again be smaller than that of HCN. Further, oxygen and sulfur do not have as large a difference in electronegativity, so even the individual bond dipoles are smaller than those in the other molecules. CCl4 has tetrahedral geometry. Although each of the individual C-Cl bonds is highly polar, the orientation of these bonds causes the dipoles to cancel each other out fully, yielding no overall dipole moment.

Answer 95

D Bond lengths decrease as the bond order increases, and they also decrease with larger differences in electronegativity. In this case, because both C2H2 and HCN have triple bonds, we cannot compare the bond lengths based on bond order. We must then rely on other periodic trends. The bond length decreases when moving to the right along the periodic table's rows because more electronegative atoms have shorter atomic radii. The nitrogen in HCN is likely to hold its electrons closer, or in a shorter radius, than the carbons in C2H2.

Answer 96

C Electronegative atoms bonded to hydrogen disproportionately pull covalently bonded electrons toward themselves, which leaves hydrogen with a partial positive character. That partial positive character is attracted to nearby negative or partial negative charges, such as those on other electronegative atoms.

Answer 97

C First recall that ammonium is NH4+ while ammonia is NH3. Ammonium is formed by the association of NH3, an uncharged molecule with a lone pair on the nitrogen, with a positively charged hydrogen cation. In other words, NH3 is a Lewis base, while H+ is a Lewis acid. This type of bonding between a Lewis acid and base is a coordinate covalent bond.

Answer 98

C All atoms in the third period or greater have d-orbitals, which can hold an additional 10 electrons. The typical "octet" electrons reside in s and p-orbitals, but elements in period 3 or higher can place electrons into these d-orbitals.

Answer 99

C All of the listed types of forces describe interactions between different types of molecules. However, noble gases are entirely uncharged and do not have polar covalent bonds, ionic bonds, or dipole moments. Therefore, the only intermolecular forces experienced by noble gases are London dispersion forces. Although these interactions are small in magnitude, they are necessary for condensation into a liquid.

Answer 100

C The reaction in this question shows a water molecule, which has two lone pairs of electrons on the central oxygen, combining with a free hydrogen cation. The resulting molecule, H3O+ has formed a new bond between H+ and H2O. This bond is created via the sharing of one of oxygen's lone pairs with the free H+ ion. This represents the donation of a shared pair of electrons from a Lewis base (H2O) to a Lewis acid (H+, electron acceptor). This type of bond is called a coordinate covalent bond.

Answer 101

C NH3 has three hydrogen atoms bonded to the central nitrogen, which also has a lone pair. These four groups (three atoms, one lone pair) lead NH3 to assume tetrahedral electronic geometry yet trigonal pyramidal molecular geometry. The nitrogen in ammonia is sp3-hybridized. By hybridizing all three p-orbitals and the one s-orbital, four groups are arranged about the central atom, maximizing distances between the groups to minimize the energy of the molecule with a tetrahedral configuration. In contrast, BF3 has three atoms and no lone pairs, resulting in sp2-hybridization. Its shape is called trigonal planar.

Answer 102

B This answer requires an understanding of the trends that cause higher or lower bond energies. Bonds of high energy are those that are difficult to break. These bonds tend to have more shared pairs of electrons and, thus, cause a stronger attraction between the two atoms in the bonds. This stronger attraction also means that the bond length of a high-energy, high-order bond such as a triple bond is short than that of its lower-energy counterparts such as single or double bonds.

Answer 103

D Ionic compounds are composed of atoms held together by ionic bonds. Ionic bonds associate charged particles with large differences in electronegativity. Rather than forming molecules or being measured by molecular weight, ionic compounds form large arrays of ions in crystalline solids and are measured with formula weights. In ionic bonds, electrons are not really shared but rather are donated from the less electronegative atom to the more electronegative atom.

Answer 104

A Of the compounds listed, Choices B and D are covalent compounds and thus are measured in molecular weights, not formula weights. The formula weights of MgCl2 is much too high (24.3 amu / 2 x 35.5 amu = 95.3 amu per formula unit). Only KCl fits the criteria (39.1 amu + 35.5 amu = 74.6 amu).

Answer 105

A First, it is helpful to know the molar mass of one mole of H2SO4, which is found by adding the atomic weights of the atoms that constitute for molecule: (2 x 1.0g/mol H) + (1 x 32.1g/ mol S) + (4 x 16.0g/mol O) = 98.1g/mol H2SO4 Gram equivalent weight is the mass (in grams) that would release one mole of protons. Because sulfuric acid has two hydrogens per molecule, the gram equivalent weight is 98.1g divided by 2, or 49.1g.

Answer 106

A The percent composition by mass of any given element within a molecule is equal to the mass of that element in the molecule divided by the molar mass of the compound, times 100%. In this case: (C = 6) (H = 1) (O = 16) C3H6O (3x12g/mol) / (58g/mol C3H6O) = 18/29 = 2/3 = 66.7% C C2H5OH (2x12g/mol) / (46g/mol C2H5OH) = 50% C C3H8 (3x12g/mol) / (44g/mol C3H8) = 9/11 = 80% C CH3OH (12g/mol) / (32g/mol) = 3/8 = 37.5% C Note that all four of these compounds are commonly encountered on the MCAT, and you should be familiar with the structure and composition of each, including their common names.

Answer 107

B This reaction is a classic example of a neutralization reaction, in which an acid and a base react to form a salt and, usually, water. Although this reaction also fits the criteria for a double-displacement reaction in which two molecules essentially exchange ions with each other, neutralization is a more specific description of the process.

Answer 108

A In this question, you are first given the masses of both reactant used to start the reaction. To figure out what will be left over, we must first determine which species is the limiting reagent. The formula weight of Na2S: (2x23g/mol Na) + (1x32.1g/mol S) = 78.1 g/mol Na2S. The formula weight of AgNO3: (1x107.9g/mol Ag) + (1x14g/mol N) + (3x16g/mol O) = 169.9g/mol AgNO3. From this we can determine that we are given: 39. 05g Na2S / 78.1g/mol = 0.5 mol Na2S 85. 5g AgNO3 / 169.9g/mol = 0.5 mol AgNO3 Because we need two moles of AgNO3 for every mole of Na2S, AgNO3 is the limiting reagent, and the correct answer choice will be given in grams of Na2S. If 0.5 mol of AgNO3 are used up, and Na2S will be consumed at half the rate of AgNO3 (based on their mole ratio), then 0.25 mol Na2S will be used up. We then have 0.25 mol excess Na2S, which has a mass of: 0.25 mol x 78.1g/mol = 19.5g Na2S.

Answer 109

D In the reaction, there is a single-displacement, with the silver in silver oxide being replaced by the aluminum to form aluminum oxide. This single-displacement reaction also necessitates a transfer of electrons in an oxidation-reduction reaction; silver, for example, changes from the +2 oxidation state to neutral. Aluminum changes from neutral to the +3 oxidation state.

Answer 110

C Typically, both single-displacement and double-displacement reactions have two reactants that swap either one or two components between the two species. Combination reactions, on the other hand, have more reactants than products because the reactants combine together to form the product.

Answer 111

B This description characterizes a combustion reaction because a hydrocarbon acts as a fuel when reacting with oxygen. Carbon dioxide (an oxide) and water are the products of such a reaction.

Answer 112

B The equation given is unbalanced, so the first step must be to balance it: 6 CO2 + 6 H2O -(hv)-> C6H12O6 + 6O2 The theoretical yield is the amount of product synthesized if the limiting reagent is completely used up. This question therefore asks how much glucose is produced if the limiting reagent is 30 grams of water. Using the three-fraction method to solve for the mass of glucose produced gives: ``` 30.0g H2O x (1 mol H2O / 18g H2O) x (1 mol C6H12O6 / 6 mol H2O) x (180g C6H12O6 / 1 mol C6H12O6) = 50g C6H12O6 ```

Answer 113

B A limiting reagent is by definition a reactant. Because Au and H2S are products, they cannot act as limiting reagents. Next, note that the given equation is unbalanced and the first step is to balance it: Au2S3 (s) + 3 H2 (g) -> 2 Au (s) + 3 H2S (g) The problem states that 2 moles of gold(III) sulfide and 5 moles of hydrogen has are available. To use up both moles of gold(III) sulfide, 6 moles of hydrogen gas are needed because there is a 1:3 ratio between these reactants. Since only 5 moles of hydrogen gas are present, that will have to be the limiting reagent.

Answer 114

B The best electrolytes dissociate readily (have a high dissociation constant) and are ionic compounds with large amounts of cations and anions. Choice D has fewer total ions with a smaller total magnitude of charge and therefore is not as strong an electrolyte as in choice B.

Answer 115

C The simplest approach is to determine the molar mass of the empirical formula. B2H5 has a molar mass of 26.6g/mol. A molecular formula is ALWAYS a multiple of the empirical formula; doubling this quantity will result in the molar mass given in the question stem. Therefore, the compound must be B4H10.

Answer 116

D Based on the information given in the question, the rate is first-order with respect to the concentration of the first reactant; when the concentration of that reactant doubles, the rate also doubles. Because the reaction is third-order, the sum of the exponents in the rate law must be equal to 3. Therefore, the reaction order with respect to the other reactant must be 3 - 1 = 2. If the concentration of this second reactant is multiplied by 1/2, the rate will be multiplied by (1/2)^2 = 1/4.

Answer 117

D If the activation energy of the forward reaction is greater than the activation energy of the reverse reaction, then the products must have a higher free energy than the reactants. The overall energy of the system is higher at the end than it was in the beginning. The net free energy change is positive, indicating an endergonic (nonspontaneous) reaction. The terms endothermic and exothermic are associated with enthalpy (ΔH). While free energy does depend on enthalpy, it also depends on entropy (ΔS); there is not enough information in the question stem to reliably determine the sign of the entropy change of the reaction.

Answer 118

D A second-order reaction can be second-order with respect to one reactant, or first-order with respect to two different reactants. In this case, one reactant was increased by a factor of 4. If the reaction is second-order with respect to this reactant, the rate law will be rate = k[A]^2[B]^0 and the rate will increase by a factor of 16. If it is first-order with respect to this reactant and first-order with respect to another reactant, the rate will be rate = k[A]^1[B]^1, and the rate will increase by a factor of 4. We do not know which of these is the correct rate law and, thus, cannot determine the effect on the rate.

Answer 119

A By definition, zero-order reactions are unaffected by the concentrations of any reactants in the reaction. Thus, changing the concentrations of these reactants will not affect the rate.

Answer 120

D The question asks which alteration does NOT affect the rate of the reaction. Temperature directly affects the rate constant (k). Changing the partial pressure of a gas will affect the number of effective collisions per time. This will make Choice B incorrect but do note that concentration changes will not affect the rate of zero-order reactions. Solvents affect the rate of reactions depending on how the reactants interact with the solvent. Removing the product of an irreversible reaction should not affect the rate of the reaction because the rate law does not depend on the concentration of products.

Answer 121

D While increasing the concentration of reactants can alter the reaction rate in first or higher-order reactions, saturated solutions containing a catalyst have a maximum turnover rate and cannot increase the rate constant or the reaction rate any higher by adding more reactant molecules.

Answer 122

A If the sum of the exponents (orders) of the concentrations of each species in the rate law is equal to 2, then the reaction is second-order. The exponents in the rate law are unrelated to stoichiometric coefficients, so NO2 and Br2 could have any stoichiometric coefficients in the original reaction and still be a second-order reaction, invalidating statement II. Statement III is incorrect because the rate can be affected by a wide variety of compounds. A catalyst, for example, could increase the rate.

Answer 123

B By definition, a catalyst increases the rate of a reaction by lowering the activation energy, making it easier for both the forward and reverse reactions to overcome this energy barrier. Catalysts are neither used up in the reaction, nor do they alter the equilibrium of a reaction. Finally, catalysts stabilize the transition state by lowering its energy, not raising it.

Answer 124

C The overall order of a reaction is the sum of the individual orders in the reaction. Therefore, the rate order is 0 + 2 + 1 = 3.

Answer 125

D To answer this question, recall that the slow step of a reaction is the rate-determining step. The rate is ALWAYS related to the concentrations of the reactants in the rate-determining step (NOT the overall reaction), so NO2 is the only compound that should be included in the correct answer. The concentration of NO2 is squared in the rate law because the question stem tells us that the system obeys second-order kinetics.

Answer 126

C This scenario likely describes a situation in which a reaction has reached equilibrium very far to the right (with high product concentration and low reactant concentration). This reaction must be reversible because the reaction did not proceed all the way to the right. Any reaction in equilibrium has equal forward and reverse rates of reaction.

Answer 127

A Carbon dioxide gas evolves and leaves the bottle, which decreases the total pressure of the reactants. Le Châtelier's principle explains that a decrease in pressure shifts the equilibrium to increase the number of moles of gas present. This particular reaction will shift to the left, which in turn will decrease the amount of carbonic acid and increase the amount of carbon dioxide and water. Oxygen and nitrogen are not highly reactive and are unlikely to combine spontaneously with carbon dioxide or carbonic acid.

Answer 128

A The larger the value of Keq (whether Kc or Kp), the larger the ratio of products to reactants. Therefore, if Kc >> 1, there are significantly larger concentrations of products than reactants at equilibrium. Even with a large Keq, the reaction will ultimately reach equilibrium far toward the products side and is therefore reversible.

Answer 129

B Adding sodium acetate increases the number of acetate ions present. According to Le Châtelier's principle, this change will push this reaction to the left, resulting in a decrease in the number of free H+ ions. Because pH is determined by the hydrogen ion concentration, a decrease in the number of free protons will increase the pH. An acid's Ka (which is simply the Keq for acid dissociation) will remain constant under a given temperature and pressure.

Answer 130

A Both increasing the pressure of the container and decreasing the volume would favor the side with fewer moles of gas, which is the product side. The significance of decreasing the volume of the container in most equilibria is that there is an increase in pressure; in this case, however the pressure remains constant despite the change in volume (choice D).

Answer 131

C An exothermic reaction produces heat. Decreasing the temperature favors product formation, resulting in an increase in the forward reaction rate with a concomitant decrease in the reverse reaction rate.

Answer 132

B The equilibrium of a reaction can be changed by several factors. Adding or subtracting heat would shift the equilibrium based on the enthalpy change (ΔH) of the reaction. Increasing reactant concentrations would shift the equilibrium in the direction of the product, and the opposite would occur if reactant concentrations were decreased. Changing the volume of a reactant would affect any reaction with gaseous reactants or products. While adding or removing a catalyst would change the reaction rates, it would not change where the equilibrium lies.

Answer 133

B At extremely high temperatures, reactants or products may decompose, which will affect the equilibrium and potentially destroy the desired products. Choice A implies that reactions have limits, which is true; however, this does not make increasing temperature unfavorable. Choice C is false because increasing temperature would also increase pressure, assuming constant volume. Choice D is incorrect because it refers to properties of irreversible reactions, which would not be involved in an equilibrium between products and reactants.

Answer 134

B Statement I is false because the addition of a catalyst could increase the rate constants of both the forward and reverse reactions. Statement II is true because in order for products to come into existence, reactants must be used up. Statement III is also true; all K values are temperature-dependent.

Answer 135

C Ka is equal to the ratio of products to reactants, with each species raised to its stoichiometric coefficient. A compound with a Ka greater than 10^-7 contains more [H+] cations than {HA-] anions at equilibrium, which makes it an acid. This means that the compound in question is likely to react with a compound that is basic. Of the four answer choices, NH3 is the only base.

Answer 136

C Reaction 2 is the reverse of reaction 1. This means that Keq for reaction 2 is the inverse of Keq of reaction 1, so the answer is 1/0.1 = [ 1 / (1/10) ] = 10.

Answer 137

A A negative ΔH value indicates an exothermic reaction, meaning that the forward reaction produces heat. Visualize this as follows: A + B <-> C + D + heat This means that removing heat by decreasing the temperature is similar to removing any other product of the reaction. To compensate for this loss, the reaction will shift to the right, causing an increase in concentration of C and D, as well as a decrease in the concentrations of A and B.

Answer 138

A This process may be adiabatic. Given that the gas was cooled, it did not maintain constant temperature (Choice C). Isobaric and isovolumetric processes appear are horizontal and vertical lines in pressure-volume graphs, respectively. Adiabatic processes appear hyperbolic on pressure-volume graphs, as described.

Answer 139

D There is not enough information in the problem to determine whether or not the reaction is spontaneous. If the signs of enthalpy and entropy are the same, the reaction is temperature dependent according to ΔG = ΔH - TΔS. Without the temperature, we cannot determine if this reaction is spontaneous, nonspontaneous, or at equilibrium.

Answer 140

D Solve this question using the equation ΔGrxn = -RTlnKeq. ΔGrxn is negative (as it must be for a spontaneous reaction), and R and T are always positive. Therefore, ln Keq must also be positive for the sign convention to work out correctly. Since ln (1) = 0, the natural logarithm of any number less than 1 will be negative. In order for ln Keq to be a positive number, Keq must be greater than 1.

Answer 141

D Combustion often involves the reaction of a hydrocarbon with oxygen to produce carbon dioxide and water. Longer hydrocarbon chains yield greater amounts of combustion products and release more heat in the process - that is, the reaction is more exothermic. Of the hydrocarbons listed here, n-pentane is the longest chain.

Answer 142

A At first glance, this might seem like a math-heavy problem, but it really doesn't require any calculations at all. We just have to keep track of which bonds are broken and which bonds are formed. Remember, breaking bonds requires energy, while forming bonds releases energy. Two bonds are broken: a C-O bond between the carbonyl carbon and oxygen of acetic acid, and an O-H bond between the hydroxyl oxygen and hydrogen of methanol. Two bonds are also formed: a C-O bond between the carbonyl carbon and the oxygen of methyl acetate, and an O-H bond between a hydroxyl group and a hydrogen to form water. Given that the same two bonds are broken and formed in this reaction, the energy change must be 0 kJ/mol.

Answer 143

C Standard temperature and pressure indicate 0 degrees Celsius and 1 atm. Gibbs free energy is temperature dependent, but if a reaction is at equilibrium, ΔG = 0

Answer 144

D There is not enough information available to determine the free energy of this reaction. While the entropy is clearly increasing (there are more particles in the system), it is unclear what the enthalpy change is. Because bonds are breaking, the reaction should be endothermic, meaning that both ΔS and ΔH are positive. In this case, it is a temperature-dependent process, and - without a temperature given - we cannot determine the sign on ΔG.

Answer 145

A A reaction with a negative enthalpy is, by definition, exothermic. Because both enthalpy and entropy are negative, this is a temperature-dependent process, and the reaction will be both endergonic and exergonic - but only at particular temperatures.

Answer 146

C For a process to progress forward spontaneously, Q must be less than Keq and will therefore have a tendency to move in the direction toward equilibrium. A spontaneous reaction's free energy is negative by convention.

Answer 147

B A calorimeter measures heat transfer. Although calorimeters often incorporate thermometers, the thermometer itself only tracks temperature, not the heat transfer itself. Choice C is irrelevant; barometers measures changes in pressure. Choice D is also incorrect because volumetric flasks measure quantitie3s of liquid, not the heat capacity of the liquid.

Answer 148

C Gases have the highest entropy, and solids have the lowest. Therefore, a phase change from a gas to a solid - deposition - would have the largest decrease in entropy of any phase change process.

Answer 149

A In an explosion, a significant amount of heat energy is released, meaning that the reaction is exothermic (ΔH < 0). The entropy change associated with an explosion is positive because energy is dispersed over a much larger area. If this is true, the expression ΔH - TΔS must be negative, indicating that this is an exergonic process (ΔG < 0). Absolute temperature can never be negative.

Answer 150

A Gases deviate from ideal behavior at higher pressures and lower volumes and temperatures, all of which force molecules closer together. The closer they are, the more they can participate in intermolecular forces, which violates the definition of an ideal gas. At low temperatures, the kinetic energy of the particles is reduced, so collisions with other particles or the wall of the container are more likely to result in significant changes in kinetic energy.

Answer 151

D Density equals mass divided by volume. The mass of 1 mole of neon gas equals 20.2 grams. At STP, 1 mole of neon occupies 22.4L. Density = p = mass/volume = 20.2g/22.4L = 10/11 = 1g/L (actual is 0.902g/L).

Answer 152

D Graham's law of effusion states that the relative rates of effusion of two gases at the same temperature and pressure are given by the inverse ratio of the square roots of the masses of the gas particles. In other words, a gas with a higher molar mass will leak more slowly than a gas with a lower molar mass. Both neon and oxygen will leak at slower rates than helium because they both have more mass than helium.

Answer 153

B Ideal gases are said to have no attractive forces between molecules. While each particle within the gas is considered to have negligible volume, ideal gases as a whole certainly do have a measurable volume. Gases have molar masses.

Answer 154

A The average kinetic energy is directly proportional to the temperature of a gas in kelvins. The kinetic molecular theory states that collisions between molecules are elastic and thus do not result in a loss of energy. Gas particles are assumed to take up negligible space in kinetic molecular theory. While the average kinetic energy of any gas as a whole is the same at a given temperature, the particles themselves have a distribution of speeds (as seen in the Maxwell-Boltzmann distribution curve).

Answer 155

B Gases are easily compressible because they travel freely with large amounts of space between molecules. Because gas particles are far apart from each other and in rapid motion, they tend to take up volume of their container. Many gases exist as diatomic molecules, but this is not a property that characterizes all gases.

Answer 156

B We will use Charles's law. First, we must convert the temperature to kelvins by adding 273 to get 300K as the initial temperature. Think of this as a proportionality: If the volume is multiplied by 3/2, the temperature will also have to be multiplied by 3/2. Thus the final temperature is 450K, which represents a 150K increase (which is equivalent to an increase of 150 degrees Celsius).

Answer 157

B The partial pressure of each gas is found by multiplying the total pressure by the mole fraction of the gas. Because 80% of the molecules are nitrogen, the mole fraction of nitrogen gas is equal to 0.80. Similarly, for helium, the mole fraction is 0.20. To find the pressure exerted by nitrogen, multiply the total pressure (150 torr) by 0.80 to obtain 120 torr of nitrogen. The remainder, 30 torr, is attributable to helium.

Answer 158

C Both a change in temperature and a change in volume can affect a gas's pressure. So if one of those two variables is kept constant, we'll definitely be able to predict which way the pressure will change. At a constant volume, heating the gas will increase its pressure, and cooling the gas will decrease it. What about when both temperature and volume are changing? If both changes have the same effect on pressure, then we can still predict which way it will change. Cooling the has and increasing its volume both decrease pressure. Choice C, on the other hand, presents too vague a scenario for us to predict definitively the change in pressure. Heating the gas would amplify the pressure, while increasing the volume would decrease it. Without knowing the magnitude of each influence, it's impossible to say whether the pressure would increase, decrease, or stay the same.

Answer 159

C Initially the concentration of the gas is decreased to one-half its original value. Recall that concentration (solubility) and partial pressure are directly related - as one increases, the other increases. If the experimenters then quadruple the partial pressure of oxygen in the vessel, the solubility is also increased by a factor of four. One-half times four gives twice the original concentration value. Misreading the answer choices as being related to the concentration before the experimenters increased the partial pressure leads to Choice D.

Answer 160

D All three choices can make a solution as long as the two components create a mixture that is of uniform appearance (homogenous). Hydrogen in platinum is an example of a gas in a solid. Brass and steel are examples of homogenous mixtures of solids. The air we breathe is an example of a homogenous mixture of gases; while these are more commonly simply referred to as mixtures, they still fit the criteria of a solution.

Answer 161

B Benzene and toluene are both organic liquids and have very similar properties. They are both nonpolar and are almost exactly the same size. Raoult's law states that ideal solution behavior is observed when solute-solute, solvent-solvent, and solute-solvent interactions are all very similar. Therefore, benzene and toluene in solution will be predicted to behave as a nearly ideal solution.

Answer 162

C Melting point depresses upon solute addition. Solute particles interfere with lattice formation, the highly organized state in which solid molecules align themselves. Colder-than-normal conditions are necessary to create the solid structure.

Answer 163

D The first step will most likely be endothermic because energy is required to break molecules apart. The second step is also endothermic because the intermolecular forces in the solvent must be overcome to allow incorporation of solute particles. The third step will most likely be exothermic because polar water molecules will interact with the dissolved ions, creating a stable solution and releasing energy.

Answer 164

C CaS will cause the most negative contribution to ΔSsoln through hydration effects because the Ca2+ and S2- ions have the highest charge density compared to the other ions. All of the other ions have charges of +1 or -1, whereas Ca2+ and S2- each have charges with a magnitude of 2.

Answer 165

B Formation of complex ions between silver ions and ammonia will cause more molecules of solid AgCl to dissociate. The equilibrium is driven toward dissociation because the Ag+ ions are essentially being removed from solution when they complex with ammonia. This rationale is based upon Le Châtelier's principle, stating that when a chemical equilibrium experiences a change in concentration, the system will shift to counteract the change.

Answer 166

B The mass percent of a solute equals the mass of the solute divided by the mass of the total solution times 100%. Plug in the values given for sucrose, the volume of water and the density of water to determine the mass% of sucrose. % mass of sucrose = (100g sucrose)/ [(300mL H2O) (0.975g/mL) +100g Sucrose] x 100% = 100/(300+100) x 100% = 100/400 x 100% = 25% Keep in mind that in rounding while calculating, the denominator was estimated to be larger than the actual value, thus giving an answer that is slightly lower than the actual value. Thus the correct answer is 25.5%. Choice A results if rounding error is not taken into account. While these answers are very close, the mass of the water must be slightly less than 300g, given the density value, so the percent composition of sucrose must be slightly higher than 25%. If the solute's mass is not added to the solvent's, the calculated value is 34.2%. Choice C neglects both the addition step and the rounding error.

Answer 167

A Mixtures that have a higher vapor pressure than predicted by Raoult's law have stronger solvent-solvent and solute-solute interactions than solvent-solute interactions. Therefore, particles do not want to stay in solution and more readily evaporate, creating a higher vapor pressure than an ideal solution. Two liquids that have different properties, like hexane (hydrophobic) and ethanol (hydrophilic, small) would not have many interactions with each other and would cause positive deviation; i.e. higher vapor pressure. Choices B and C are composed of liquids that are similar to one another and would not show significant deviation from Raoult's law. Choice D contains two liquids that would interact very well with each other, which would actually cause a negative deviation from Raoult's law - when attracted to one another, solutes and solvents prefer to stay in liquid form and have a lower vapor pressure than predicted by Raoult's law.

Answer 168

A Dissolution is governed by enthalpy and entropy, which are related by the equation ΔGsoln = ΔHsoln - TΔSsoln. The cooling of the solution indicates that heat is used up in this bond-breaking reaction. In other words, dissolution is endothermic, and ΔH is positive. The reaction is occurring spontaneously, so ΔG must be negative. The only way that a positive ΔH can result in a negative ΔG is if the entropy, ΔS, is a large, positive value. Conceptually, that means that the only way the solid can dissolve is if the increase in entropy is great enough to overcome the increase in enthalpy. Choice B is incorrect because it is clearly stated in the question stem that KCl dissolves; further, all salts of Group 1 metals are soluble. Choice C is incorrect because ΔSsoln must be positive in order for KCl to dissolve. Finally, Choice D is incorrect because solute dissolution would cause the boiling point to elevate, not depress. It is also not a piece of evidence that could be found simply by observing the beaker's temperature change.

Answer 169

D Since both salts have a formula MX3 (one of one particle, three of another), it is possible to directly compare the molar solubilities of each. When the solutions are mixed, [OH-] is above saturation levels for both the cobalt and the thallium in the solution. Since thallium (III) hydroxide has a smaller Ksp than that of cobalt (III) hydroxide, it will react first. The ion product of the mixed solution is higher than the Ksp for thallium (III) hydroxide, and the system will shift left to precipitate solid thallium (III) hydroxide. After the thallium (III) hydroxide precipitates, a small excess of [OH-] will remain, which gives an ion product slightly above the Ksp of cobalt (III) hydroxide. This will cause a small amount (1%-3%) of cobalt (III) hydroxide to also precipitate.

Answer 170

D A Bronsted-Lowry base is defined as a proton acceptor. Ammonia, fluoride, and water each accept a proton. HNO2, is a far better Bronsted-Lowry acid, donating a proton to solution.

Answer 171

B First, convert the concentration to 5 x 10^-3 M. Next, because sulfuric acid is a strong acid, we can assume that, for the majority of sulfuric acid molecules (although not all all), both protons will dissociate. The concentration of hydrogen ions is therefore 2 x (5 x 10^-3), or 10^-2. The equation for pH is pH = -log{H+]. If [H+} = 10^-2 M, then pH = 2

Answer 172

A Answering this question is simply a matter of knowing nomenclature. Acids ending in -ic are derivatives of anions ending in -ate, while acids ending in -ous are derivatives of anions ending in -ite. ClO3- is chlorate because it has more oxygen than the other commonly occurring ion, ClO2-, which is named chlorite. Therefore, HClO3 is chloric acid. HClO2 represents chlorous acid. HClO represents hypochlorous acid.

Answer 173

B Soluble hydroxides of Group IA and IIA metals are strong bases. Choices B and C are both weak bases; however, methylamine contains an alkyl group, which is electron-donating. This increases the electron density on the nitrogen in methylamine, making it a stronger (Lewis) base. Therefore, ammonia is the weakest base.

Answer 174

B The purpose of a buffer is to resist changes in the pH of a reaction. Buffers are not generally used to affect the kinetics of a reaction. Choice A is correct only in specific circumstances where the pH of the buffer solution itself is neutral. Many natural buffer systems maintain pH in the acidic or basic ranges.

Answer 175

D The question is asking for pH, but because of the information given, we must first find the pOH and then subtract it from 14 to get the pH. Use the Henderson-Hasselbalch equation: pOH = pKb + log([conjugate acid] / [base]) = 3.45 + log(70mM / 712mM) = 3.45 + log(1 / 10) = 3.45 - 1 = 2.45 If the pOH = 2.45, then the pH = 14 - 2.45 = 11.55

Answer 176

B Gram equivalent weight is the weight (in grams) that releases 1 acid or base equivalent from a compound. Because H3PO4 contains 3 protons, we find the gram equivalent weight by dividing the mass of one mole of the species by 3. The molar mass of phosphoric acid is 98g/mol, so the gram equivalent weight is 32.7g.

Answer 177

This question requires application of the acid dissociation constant. Weak acids do not dissociate completely; therefore, all three species that appear in the balanced equation will be present in solution. Hydrogen ions and conjugate base anions dissociate in equal amounts, so [H+] = [XO2-]. If the initial concentration of HXO2 was 2M and some amount x dissociates, we will have x amount of H3O+ and XO2- at equilibrium, with 2M - x amount of HXO2 at equilibrium. Note that x was considered negligible when added or subtracted, per usual. Solving for x, we get: x^2/2 = 3.2 x 10^-5 -> x^2 = 6.4 x 10^-5 = 64 x 10^-6 = 8 x 10^-3

Answer 178

C A higher Ka implies a stronger acid. Weak acids usually have a Ka that is several orders of magnitude below 1. The pKa of a compound is the pH at which there are equal concentrations of acid and conjugate base; the pKa of this compound would be -log 1 = 0. With such a low pKa, this compound must be an acid. Therefore, the pH of any concentration of this compound must be below 7.

Answer 179

D An amphoteric species is one that can act either as an acid or a base, depending on its environment. Proton transfers are classic oxidation-reduction reactions. Choice C is true because many amphoteric species, such as water and bicarbonate, can either donate or accept a proton. Choice D is false, and thus the correct answer because amphoteric species can be either polar or nonpolar in nature.

Answer 180

C NaOH is a strong base; as such, there will be 1.2 x 10^-5 M OH- in solution. Based on this information alone, the pOH must be between 4 and 5, and the pH must be between 9 and 10. Using the shortcut, pOH = 5 - 0.12 = 4.88. pH = 14 - pOH = 14 - 4.88 = 9.12 (actual 9.08).

Answer 181

C Use the equivalence point equation: NaVa = NbVb. Ba(OH)2 can dissociate to give two hydroxide ions, so its normality is 2 M x 2 = 4. H3PO4 can dissociate to give three hydronium ions, so its normality is 6 M x 3 = 18N. Plugging into the equation, we get (18N)(4L) = (4N)(Vb). Therefore, Vb is 18L.

Answer 182

C The oxidizing agent is the species that is reduced in any given equation. In this problem, six hydrogen atoms with +1 oxidation states in NH3 are reduced to three neutral H2 molecules.

Answer 183

B First, balance the atoms in the equation: Cr2O7^2- + 14H+ -> 2Cr2+ + 7H2O Now, adjust the number of electrons to balance the charge. Currently, the left side has a charge of +12 (-2 from dichromate and +14 from protons). The right side has a charge of +4 (+2 from each chromium cation). To decrease the charge on the left side from +12 to +4, we should add 8 electrons: CrO7^2- + 14H+ + 8e- -> 2Cr2+ + 7H2O

Answer 184

A Hydride ions are composed of a hydrogen nucleus with two electrons, thereby giving it a negative charge and a considerably tendency to donate electrons. LiAlH4 is therefore a strong reducing agent. Strong reducing agents tend to have metals or hydrides; strong oxidizing agents tend to have oxygen or a similarly electronegative element.

Answer 185

C In NaClO (sodium hypochlorite), sodium carries its typical +1 charge, and oxygen carries its typical +2 charge. This means that the chlorite atom must carry a +1 charge in order to balance the overall charge of zero.

Answer 186

D A net ionic equation represents each of the aqueous ions comprising the reactants and products as individual ions, instead of combining them as formula units. Thus, Choice A is not a net ionic reaction. The term net means that the correct answer does not include any spectator ions (ions that do not participate in the reaction). In this reaction, nitrate (NO3-) remains unchanged. Therefore, Choices B and C are incorrect.

Answer 187

C What you are shown is a new ionic equation. If two moles of FeSCN are created, two moles of Fe3+ must be used because the mole ratio is 1:1. Iron sulfate has the formula Fe2(SO4)3 because sulfate has a charge of -2 and iron has a charge of +3 (based on the new ionic equation). Therefore, one mole of iron sulfate is needed to make two moles of iron for the reaction. The molar mass of iron sulfate is 2 x 55.8g/mol + 3 x 32.1g/mol + 12 x 16.0g/mol =399.9g/mol This most closely matches Choice C. The most common error would be to calculate the amount of iron, which would be 111.6g.

Answer 188

D When assigning oxidation numbers, one starts with elements of known oxidation state first, and determines the oxidation state of the other elements by deduction. As a noble gas, argon, will always have an oxidation state of 0. As a Group VIIA element, fluorine, will always have an oxidation state of 0 (by itself) or -1 (in a compound). As a Group IIA element, strontium will have an oxidation state of 0 (by itself) or +2 (in a compound). Like most transition metals, iridium, can have various oxidation states, ranging from -3 to +8. Therefore, one would have to determine the oxidation states of other atoms in an iridium-containing compound to determine iridium's oxidation number.

Answer 189

A The formula for methanol is H3COH, for methanal is HCHO, and for methanoic acid is HCOOH. If we assign oxidation numbers to carbon in each molecule, it starts at -2, then becomes 0, then becomes +2. In general, it is often easier to think of oxidation as a gain of bonds to oxygen (or a similarly electronegative element) or loss of bonds to hydrogen for organic compounds. Therefore, because the carbon is oxidized as one converts from an alcohol to an aldehyde to a carboxylic acid, the oxidation number must increase.

Answer 190

C Start with the atoms that have oxidation states of which you are certain. Potassium is a Group IA metal, and therefore must have an oxidation state of +1. Hydrogen is almost always +1, unless it is paired with a less electronegative element (which is not the case here). Oxygen is generally -2. Because there are four oxygens, they create a total negative charge of -8 which is partially balanced by two hydrogens (+2) and potassium (+1). Therefore, phosphorus has a +5 charge, making it the highest oxidation state.

Answer 191

B Recall that oxygen has an oxidation state of -2. Therefore, in tungsten(IV) oxide, Choice A, tungsten has an oxidation state of +4. In tungsten(VI), Choice B, it has an oxidation state of +6. In tungsten(III), Choice C, it is +3. In tungsten pentoxide, Choice D, it is +5.

Answer 192

A Step I is a disproportionation reaction because chlorine starts with an oxidation state of 0 in the reactants and ends up with an oxidation state of +1 in HOCl and -1 as Cl-. In the other reactions, no element appears with different oxidation states in two different products. Therefore, only Step I is a disproportionation reaction.

Answer 193

C Potentiometry refers to carrying out an oxidation-reduction titration with a voltmeter present to get precise readings of the reactions electromotive force (emf) to determine the endpoint. This is analogous to using a pH meter in an acid-base titration because it uses technology to get precise readings for plotting a titration curve. Indicators can be used in both acid-base and redox titrations, but provide a qualitative (rather than quantitative) analysis of the titration. Oxidizing and reducing agents are used in redox titrations, not acid-base titrations.

Answer 194

C In the oxidation-reduction reaction of a metal with oxygen, the metal will be oxidized (donate electrons) and oxygen will be reduced (accept electrons). A species with a higher reduction potential is more likely to be reduced, and a species with a lower reduction potential is more likely to be oxidized. Based on the information in the question, iron is oxidized more readily than those metals; this means that iron has a lower reduction potential.

Answer 195

B To determine the standard electromotive force of a cell, simply subtract the standard reduction potentials of the two electrode. In this case, the cathode is zinc because it is being reduced; the anode is silver because it is being oxidized. Thus, Ecell = Ered(cathode) - Ered(anode) = -0.763 - 0.337 = -1.10V While we must multiply the silver half-reaction by two to balance electrons, the actual value for the reduction potential does not change. Remember that the standard reduction potential is determined by the identity of the electrode, not the amount of it present.

Answer 196

A Oxidation occurs at the anode, and reduction occurs at the cathode. Because Cu is the anode, it must be oxidized. The reduction potential of the cathode cannot be less than that of the anode for a galvanic cell. Therefore, mercury must be the cathode. In a concentration cell, the same material is used as both the cathode and anode; however, this question assumes equal concentrations. If both electrolyte solutions have the same concentration, there will be no oxidation-reduction reaction and, therefore, no anode or cathode.

Answer 197

C In an electrolytic cell, ionic compounds are broken up into their constituents; the cations (positively charged ions) migrate toward the cathode, and the anions (negatively charged ions) migrate toward the anode. In this case, the cations are H+ ions (protons), so option I is correct. Electrons flow from anode to cathode in all types of cells, meaning that option III is also correct. Option II is incorrect for two reasons. First, it is unlikely that the anions in any cell would be O^2- rather than OH-. Second, and more significantly, these anions would flow to the anode, not the cathode.

Answer 198

D This answer comes directly from the equation relating Gibbs free energy and Ecell. ΔG = -nFEcell, where n is the number of moles of electrons transferred and F is the Faraday constant, 96,485 C/mol e-. To determine n, one must look at the balanced half-reactions occurring in the oxidation-reduction reaction.

Answer 199

B Salt bridges contain inert electrolytes. Ionic compounds such as Choices A, C, and D, are known to be strong electrolytes because they completely dissociate in solution. Choice B cannot be considered an electrolyte because its atoms are covalently bonded and will not dissociate in aqueous solution. Choices B and C may appear similar, but there is an important distinction to be made. Choice C implies that Mg^2+ and SO3^2- are the final, dissociated ionic constituents, while Choice B implies that neutral SO3 would have to be dissolved in solution.

Answer 200

B Potential, as measured by Ecell, is dependent only on the identity of the electrodes and not the amount present. Similarly, the equilibrium constant depends on only the identity of the electrolyte solutions and the temperature. However, as the electrode material is increased, the surface area participating in oxidation-reduction reactions is increased and more electrons are released, making statement II correct.

Answer 201

D Ecell is dependent upon the change in free energy of the system through the equation RT ln Keq = nFEcell. The temperature, T, appears in this equation: thus, a change in temperature will impact the Ecell.

Answer 202

B If this were a galvanic cell, the species with the more positive reduction potential (cadmium) would be reduced. The cathode is always reduced in an electrochemical cell, so sodium could not be the cathode in such a galvanic cell. Sodium would be the cathode in an electrolytic cell, however, which would make cadmium the anode. Note that we do not have to determine Ecell because we already know the answer. However, the Ecell would be -2.71 - (-0,40) - -2.31V for an electrolytic cell and +2.31V for a galvanic cell.

Answer 203

D A spontaneous electrochemical reaction has a negative ΔG. Using the equation ΔG = -RT ln Keq, Keq > 1 would result in ln Keq > 0, which means ΔG < 0. A negative electromotive force or equilibrium state would not correspond to a spontaneous reaction. Concentration cells can be spontaneous; however, if the concentration cell has reached equilibrium, it would cease to be a spontaneous reaction. When an answer choice may be true, but does not have to be - it is the wrong answer on Test Day.

Answer 204

A A change in pH has a direct correlation to the hydrogen ion (H+) concentration. Decreasing the pH increases the H+ concentration, which means the concentration of products has increased in the oxidation of sulfur dioxide. This means it would be harder to liberate electrons, thus decreasing the emf. One could also view this decrease in oxidation potential as an increase in reduction potential. If Ered(anode) increases, then Ecell must decrease according to Ecell = Ered(cathode) - Ered(anode).

Answer 205

C An electrolytic cell is nonspontaneous. Therefore, the ΔG must be positive and Ecell must be negative. The change in entropy may be positive or negative, depending on the species involved. According to the equation ΔG = -RT ln Keq, Keq < 1 would result in ln Keq < 0, which means ΔG > 0.

Answer 206

D Compared to other cell types, lead-acid batteries have a characteristically low energy density. While Choice A is a true statement, the incomplete dissociation of sulfuric acid does not fully explain the low energy density of lead-acid batteries. Choice C is likely to be an opposite; the more easily the electrodes dissociate, the easier it is to carry out oxidation-reduction reaction with them.

Answer 207

C During the recharge cycle, Ni-Cd cells will accept current from an outside source until the Cd and NiO(OH) electrodes are pure; at this point, the reaction will stop because Cd(OH)2 runs out and no more electrons can be accepted. Choices A and B are both true statements, but they fail to explain why overcharging the battery (continuing to try to run current into the battery even when the electrodes are reverted to their original state) is not a problem with Ni-Cd batteries. Finally, surge current refers to the initial burst of current seen in some batteries; once charged, the surge current will not increase even if the power sources continues to be run because no additional charge will be stored on the electrodes.