MCAT Chemistry Flashcards
Many double-displacement reactions are enzyme-catalyzed via the “ping-pong” mechanism, so called because the reactants appear to bounce off the enzyme like a ping pong ball. These reactions typically have two reactants and two products. In a third-order reaction involving two reactants and two products, doubling the concentration of the first reactant causes the rate to increase by a factor of 2. What will happen to the rate of this reaction if the concentration of the second reactant is cut in half?
A) It will increase by a factor of 2
B) It will increase by a factor of 4
C) It will decrease by a factor of 2
D) It will decrease by a factor of 4
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.
For the most part, biological pH is slightly basic. However, the stomach is very acidic, and its contents must be swiftly neutralized by basic bicarbonate ions upon entering the small intestine, which has a basic pH. How many liters of 2M Ba(OH)2 are needed to titrate a 4L solution of 6M H3PO4?
A) 1.33L
B) 12L
C) 18L
D) 56L
C
Use the equivalence point equation:
(N1)(V1) = (N2)(V2)
Ba(OH)2 can dissociate to give two hydroxide ions, so its normality is 2M x 2 = 4N. H3PO4 can dissociate to give three hydronium ions, so its normality is 6M x 3 = 18N. Plugging into the equation, we get (18N)(4L) = (4N)(V2). Therefore, V2 is 18L.
The body’s pH is tightly regulated because specific enzymes function best within a narrow pH range. What is the approximate pH of a 1.2 x 10^-5 M aqueous solution of NaOH?
A) 4.92
B) 7.5
C) 9.08
D) 12.45
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 = 9.12 (actual = 9.08).
For some compounds, the empirical and molecular formula are identical. H2O is an example of this. Which of the following molecules does not have the empirical formula CH?
A) Benzene B) Ethyne C) Acetylene D) Toluene E) None of the above
D
Benzene has a molecular formula of C6H6, making its empirical formula CH. Ethyne has a molecular formula of C2H2, making its empirical formula CH as well. Acetylene is just another name for ethyne. Toluene has a molecular formula of C7H8, which is also its empirical formula, not CH.
Which of the following compounds has the highest boiling point?
A) Heptane
B) Octane
C) 2-methylheptane
D) 2,2-dimethylhexane
B
The boiling point of hydrocarbons increases as the number of carbons increases. Therefore, heptane has a higher boiling point than hexane, which has a higher boiling point than pentane. This is because the intermolecular forces between molecules keep the hydrocarbons in solution, requiring more heat to overcome these forces and become a gas. In the case of branched molecules, the hydrocarbons are not able to “stack” as easily as straight-chain hydrocarbons. This reduces intermolecular interactions and, subsequently, the boiling point. Even though octane, 2,2-dimethylhexane, and 2-methylheptane have eight carbons, octane has the highest boiling point.
Which of the following best represents the relationship among the boiling points in FBr, argon, and Cl2?
A) Ar < FBr < Cl2
B) FBr < Cl2 < Ar
C) Cl2 < Ar < FBr
D) Ar < Cl2 < FBr
D
The order of boiling points corresponds to the strength of intermolecular forces. Argon has the weakest intermolecular forces: as a neutral atom, it is less polarizable than the larger Cl2. The latter has no dipole moment, so it exerts weaker forces than FBr.
Which of the following is closest to the bond angle of an sp2-hybridized carbon atom?
A) 180•
B) 140•
C) 120•
D) 110•
C
An sp2-hybridized carbon has a trigonal planar geometry. This means that the bond angle will be 120•. An sp-hybridized carbon has a linear geometry, meaning a bond angle of 180•. Finally, an sp3-hybridized carbon has a tetrahedral geometry, with bond angles of 109.5•.
Which of the following equations could be possibly used to determine the rate of diffusion of a gas in an enclosed flask once the valve is opened?
A) PE = mgh
B) KE = (1/2)mv^2
C) F = kQq/r^2
D) PV = nRT
B
The rate of expansion (or diffusion) is directly proportional to the mean velocity of the gas molecules. The only equation that has a term for velocity is B.
Which of the following is NOT characteristic of hydrogen bonding?
A) The hydrogen atom involved must be covalently bonded to a very electronegative atom
B) The hydrogen bonds are typically weaker than ionic or covalent bonds
C) The other atom involved in the hydrogen bond (not the hydrogen atom) must be covalently bonded to a hydrogen atom
D) The other atom involved in the hydrogen bond (not the hydrogen atom) must possess at least one lone pair of electrons
C
Answer choice C is not necessary for the formation of a hydrogen bond. A classic example is that of ethers. Ethers are water soluble because the oxygen in ether, which is covalently bonded to two carbons, can hydrogen bond with the hydrogen in H2O. Note regarding answer choice C: the 3-D shapes of many proteins and DNA is due in part to intramolecular H-bonds.
Which of the following is the correct way to dispose of excess reagent?
A) Add the excess back into the reaction
B) Return excess to the stock bottle
C) Flush down the sink with lots of water
D) Dispose in the liquid container
D
When dealing with excess reagent, you should never pour it back into the stock solution to avoid contamination. Unless told otherwise, it is best to only drain water in the sinks of a chemistry lab. If the reagent is toxic or flammable, leaving it in an open container is not a safe practice. Therefore, the proper way to dispose of excess chemicals is to put it in a clearly marked chemical waste container.
The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?
A) H3N+-CH2-COO-
B) H2N-CH2-COOH
C) H3N+-CH2-COOH
D) H2N-CH2-COO-
A
Recall that you can use isoelectric point (PI) to predict an amino acid’s charge at any given pH. Specifically, if PI < pH, then this means that pH is high – which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower — which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is A since this is the amino acid where both charges cancel each other out, leaving a neutral zwitterion glycine.
When metallic substances lose electrons, they are trying to achieve the electron configuration of what?
A) The metal immediately before it on the periodic table
B) The metal immediately after it on the periodic table
C) The nearest halogen
D) The previous noble gas
D
A metal prefers to lose electrons so that it has the configuration of 8 around its valence shell so that it is equivalent to the stable noble gas previous to it.
Which of the following acids have the largest pKa?
A) CH3CO2H
B) FCH2CO2H
C) CF3CO2H
D) Cl3CCO2H
A
Recall that pKa = -log Ka so their relative values, because of the negative sign in the equation, are always opposite to each other. Thus in an acid base equilibrium, AH A- + H+, the largest pKa value is associated with the weakest acid. The unhalogenated carboxylic acid A is the weakest acid (acetic acid is the IUPAC name; ethanoic acid is the systematic way to name it). In the other cases, the electron withdrawing halogen substituent stabilize the carboxylate conjugate base (anion) making the parent acid more acidic.
A student creates a 1M citrate (C3H5O(COO)3 3-) buffer solution at pH 5.0. He then dilutes the solution to 200 mM, and adds it to his experiment. What is the pH of the 200 mM buffer solution?
A) Between pH 5 and pH 7
B) Greater than pH 7
C) Less than pH 5
D) The pH will remain close to 5
D
Buffers are designed to maintain pH at a certain level, and operate through the complementation of a conjugate acid and base. As long as the ratios of acid and base remain the same in the buffer system, and they will no matter what the dilution, as long as the dilution is not very very small (i.e. 1 mM or less), the pH will remain the same.
The following reaction was carried out in a closed system: N2(g) + O2(g) 2NO(g)
Hrxn = -ve value
Which of the following will favor the production of NO?
A) Decreased pressure
B) A catalyst
C) Increased pressure
D) Decreased temperature
D
Since the reaction is exothermic (negative enthalpy), heat can be considered a product in this reaction. A decrease in temperature will result in the equilibrium shifting to the right to bring heat to the system resulting in an increase in NO production, in accordance with Le Chatelier’s principle. A change in overall pressure of the system will not affect the equilibrium because there is an equal amount of moles of gas on both sides of the equation providing an even pressure. A catalyst affects the rate of a reaction not thermodynamic properties such as equilibrium.
Consider the following reaction:
NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq)
Keq = 1.81 x 10^-5
Which of the following can be concluded from the chemical equilibrium constant?
A) The reaction is slow to reach chemical equilibrium
B) The equilibrium mixture has more reactants than products
C) The equilibrium mixture has more products than reactants
D) The equilibrium mixture evaporates quickly leaving only products
B
The chemical equilibrium constant is calculated by dividing the concentration of the products by the concentration of the reactants at equilibrium. The magnitude of the chemical equilibrium constant can shed light on the composition of the mixture at equilibrium. Since the equilibrium constant given in the question is small, the equilibrium mixture would contain more reactants than products. If the equilibrium constant were larger than 1, this would indicate that the equilibrium mixture was made up of more products than reactants. Note: the chemical equilibrium constant is not an indicator of the rate of the reaction, only the concentrations of the equilibrium mixture.
The concentration of a reactant in a first-order decomposition reaction is decreased to 1/8 of its original value in 100 s. What is the half-life of the reactant?
A) 12.5 s
B) 25 s
C) 33.3 s
D) 50 s
C
You can test each of the answers: for 33.3 s intervals, the first 33.3 s finds the concentration dropping by 1/2, the next down to 1/4, the third down to 1/8, for a total of 100 s.
Galvanizing is a process used to protect steel by applying a sacrificial coating? Which metal is used in galvanizing?
A) Zinc
B) Gold
C) Copper
D) Aluminum
A
To provide a sacrificial coating the coating metal must be more reactive than the base metal being protected. Thus gold and copper are not applicable as they are both less reactive than iron and can only provide a passive coating.
Aluminum and zinc are more reactive than iron. Aluminum and zinc both form protective oxide layers that will stop corrosion of the iron, but aluminum is too reactive to be used in exposed weathering conditions. It is zinc that has just the right degree of reactivity that allows it to both protect the iron and not be corroded away, providing a strong and long lasting sacrificial coating.
Which of the following molecules are Lewis acids? (Me = methyl)
BF3, NH3, Me3C+ and H2O
A) BF3 and H2O
B) NH3 and Me3C+
C) BF3 and Me3C+
D) NH3 and H2O
C
A Lewis acid is an electron pair acceptor by virtue of it having an empty orbital. Both BH3 and CH3C+ are electron deficient, and therefore are Lewis acids. {Mnemonic: lEwis Acid = Electron Acceptor}
For the reaction 2NO2 –> N2O4, which of the following is true?
I. Adding NO2 shifts the equilibrium to the right.
II. Increasing the volume of the container shifts the equilibrium to the right.
III. Adding helium gas to the container shifts the equilibrium to the right.
A) I only
B) I and II only
C) I and III only
D) III only
A
The reaction shifts to the right to accommodate added NO2. Increasing the volume has the reverse effect – there’s more room for the more numerous NO2’s. Adding an inert gas raises the total pressure, but only partial pressures of reactants and products figure into the equilibrium expression.
Which of the following compounds has the greatest ionic character in its bonds? (You may consult a Periodic Table)
A) LiCl
B) CaCl2
C) BCl3
D) RbCl
D
Ionic character increases with increasing electronegativity of the potential anion and decreasing electronegativity of the potential cation. Electronegativity increases as one moves to the top right-hand corner of the periodic table, and decreases in the opposite direction.
Since the potential anion of all four compounds in question is chloride, we only need to find the compound with the cation located furthest toward the bottom left-hand corner of the periodic table. This compound is RbCl.
A cobalt electrode is immersed in 1.0M Co2+ and a lead electrode is immersed in 1.0 M Pb2+. Given are standard reduction potentials.
Co(2+) + 2e- -> Co ; E(reduction)= -0.28 V
Pb(2+) + 2e- -> Pb ; E(reduction)= -0.13 V
Which of the following describes the cell?
A) E(cell) = -0.15, galvanic
B) E(cell) = 0.15 V, galvanic
C) E(cell) = -0.41 V, electrolytic
D) E(cell) = 0.41 V, galvanic
B
The standard electrode potential E(cell) for a cell is found using the equation:
E(cell) = E(oxidation) + E(reduction)
In this case, lead will be reduced because it has a higher reduction potential than cobalt. This means that cobalt is being oxidized. Remember that on the MCAT, you will always be given standard reduction potentials, so if we need to find an oxidation potential, we simply switch to the negative reduction potential. Our equations should now look like this:
Co -> Co2+ + 2e- ; E(oxidation)= +0.28 V
Pb2+ + 2e- -> Pb ; E(reduction)= -0.13 V
Thus, using the first equation, we arrive at
E(cell) = (+0.28 V) + (-0.13 V) = 0.15 V
Remember that if E(cell) > 0, the reaction is spontaneous and happens in a galvanic cell. (The opposite is a non-spontaneous, electrolytic cell). Also, lead cobalt batteries were used in some of the first electric cars!
Which of the following best describes the process of oxidation?
A) The process in which electrons are added
B) The process in which oxygen is produced
C) The process in which hydrogen content increases
D) The process in which the oxidation number increases
D
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. The mnemonic LEO is A GERC stands for Loss Electrons Oxidation is Anode, Gain Electrons Reduction Cathode.
Which is a strong acid?
A) CH3CH2COOH
B) H2SO4
C) CH3CH2OH
D) NH4OH
B
A strong acid must be able to provide large numbers of hydrogen ions when dissolved in water, (H+(aq)). Thus a strong acid must dissociate (break into ions) completely upon dissolving in water to provide as many H+(aq) ions as possible.
All these compounds are water soluble, but only CH3CH2COOH and H2SO4 can provide H+(aq) ions. CH3CH2COOH or propanoic acid, is only partially dissociates (<1%) in water and is thus a weak acid. H2SO4 or sulfuric acid, completely (100%) dissociates into sulfate ions and hydrogen ions upon dissolving in water and is thus the strong acid of the group.
CH3CH2OH – ethanol does not dissociate, but stays as a complete molecule in solution.
NH4OH – ammonium hydroxide, a weak base which partially dissociates to give few hydroxide OH–(aq) ions.
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
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.
Which set of 1 Molar solutions has the correct pH values?
A) Water pH = 0; hydrochloric acid pH = 4; acetic acid pH = 7; ammonia solution pH = 10; sodium hydroxide solution pH = 14
B) Acetic acid pH = 0; hydrochloric acid pH = 4; water pH = 7; sodium hydroxide solution pH = 10; ammonia solution pH = 14
C) Sodium hydroxide solution pH = 0; ammonia solution pH = 4; water pH = 7; acetic acid pH = 10; hydrochloric acid pH = 14
D) Hydrochloric acid pH = 0; acetic acid pH = 4; water pH = 7; ammonia solution pH = 10; sodium hydroxide solution pH = 14
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.
A zero order reaction takes 5 minutes to decrease the original concentration of the reactants by half. How much longer will it take (in minutes) to have the concentration drop to a quarter of its original concentration?
A) 2.5
B) 5
C) 7.5
D) 10
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.)
A reaction is found to have an equilibrium constant of 600 at a temperature of 300K and a pressure of 1.0 atm. It can be concluded that at these conditions:
I. ΔH < 0 II. ΔS < 0 III. ΔG < 0
A) I only
B) II only
C) III only
D) I and II only
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.
The electron configuration of Cd2+ is best described by which of the following?
A) [Kr] 4s^25d^10
B) [Kr] 4d^10
C) [Kr] 5s^24d^10
D) [Kr] 5s^24d^8
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.
An electrolytic cell is used to reduce Ti(3+) to Ti metal. If a current of 3.5 A is passed through a solution of Ti(3+) for 2.0 min, which expression below can be used to calculate the moles of Ti deposited?
A) 3(2.0)(3.5)
B) (3.5)(2.0)(60)/(3)
C) (3.5)(2.0)(60)(3)/(96,500)
D) (3.5)(2.0)(60)/(96,500)(3)
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.
During the reaction to form ammonia from nitrogen and hydrogen according to the reaction: N2(g) + 3 H2(g) <-> 2 NH3(g), ammonia was being produced at the rate of 1.50 mol.L-1.s-1.
What were the respective rates of consumption of nitrogen and hydrogen at this time?
A) N2(g) 1.50 and H2(g) 1.50 mol.L-1.s-1
B) N2(g) 2.25 and H2(g) 0.75 mol.L-1.s-1
C) N2(g) 0.75 and H2(g) 2.25 mol.L-1.s-1
D) N2(g) 3.00 and H2(g) 1.00 mol.L-1.s-1
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.
When a catalyst is added to a chemical reaction, what will be its effect on the energy of the activated complex and on the rate of the reaction?
A) The energy of the activated complex will increase, and the reaction rate will decrease
B) The energy of the activated complex will decrease, and the reaction rate will increase
C) The energy of the activated complex and the reaction rate will increase
D) The energy of the activated complex and the reaction rate will decrease
B
A catalyst lowers the activation energy of a reaction, causing an increase in the reaction rate.
A chemist runs a reaction that results in more stable bonds and orderly arrangement of the products versus the reactants. Which of the following is true of this reaction?
A) The enthalpy and entropy changes are positive
B) The enthalpy and entropy changes are negative
C) The enthalpy change is positive, and the entropy change is negative
D) The enthalpy change is negative, and the entropy change is positive
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.
A student finds that BaCO3 is more soluble in HNO3 than in plain water. Which of the following is a likely explanation for this effect?
A) The NO3– ion attaches preferentially to the Ba2+
B) Ba2+ is a strong base and reacts completely with the H+ from the nitric acid, causing the barium salt to dissociate
C) Carbonate is a weak base and reacts with the H+ from the strong acid, causing the barium salt to dissociate
D) The HNO3– reduces the Ba2+ to barium metal
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.
The many steps of glycolysis all exist in a state of dynamic equilibrium between reactant and product concentrations. Which of the following actions does NOT affect the equilibrium position of a reaction?
A) Adding or subtracting heat
B) Adding or removing a catalyst
C) Increasing or decreasing concentrations of reactants
D) Increasing or decreasing volumes of reactants
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.
Which of the following correctly expresses both a correct statement and a correct explanation for that statement?
A) The solubility of Fe(OH)3 is pH-dependent owing to the half-cell potential of Fe(3+)/Fe(2+).
B) The solubility of Fe(OH)3 is pH-dependent owing to the presence of [OH-] in the solubility expression.
C) The solubility product of Fe(OH)3 is pH-dependent owing to the presence of [OH-] in the solubility expression.
D) The solubility product of Fe(OH)3 is not pH-dependent since [H+] does not appear in the solubility expression.
B
Note solubility, not solubility product, is hydroxide-dependent. D is wrong because [H+] and [OH-] are related.
Which of the following is closest to the pH of a mixture made up of 0.1 M each formic acid (whose Ka is 1.8 x 10-4) and potassium formate?
A) 0.1
B) 2
C) 4
D) 7
C
This solution is a buffer, for which:
[H+] = Ka [HFor]/[For–] = Ka
pH = -log (1.8 x 10^-4) = 3.7
Given that a parent and a daughter nucleus are isotopes of the same element, the ratio of alpha to beta decays which produced the daughter must be which of the following?
A) 1 to 1
B) 1 to 2
C) 2 to 1
D) 2 to 3
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.
An electron moving at constant velocity enters the area between two charged plates, as shown below. Which of the paths is most consistent with the electron’s trajectory after leaving the area between the charged plates?
+—————————- A (curved up)
-> B (Straight line up)
C (Straight line parallel)
-______________________ D (Straight line down)
(B and D are roughly 45 degrees respectively)
A) A
B) B
C) C
D) D
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).
A student wishes to find the solubility of PbCl2, whose K(sp) = 7 x 10^-29. Which equation should she solve to do so?
A) 7 x 10^-29 = (x)(x)
B) 7 x 10^-29 = (x)(2x)
C) 7 x 10^-29 = (x)(x)^2
D) 7 x 10^-29 = (x)(2x)^2
D
Note that [Cl^-] = 2x.
According to the Arrhenius equation (k = Ae^-E/RT), if a reaction occurs at room temperature, what would you expect to happen if the temperature was increased by a factor of 4?
A) The k value would increase by a factor of 4
B) The k value would not change
C) The k value would increase, but only slightly
D) The k value would decrease, but only slightly
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.
At a given temperature T in kelvin, the relationship between the three thermodynamic quantities including the change in Gibbs free energy (ΔG), the change in enthalpy (ΔH) and the change in entropy (ΔS), can be expressed as follows:
ΔG = ΔH – TΔS
The sublimation of carbon dioxide occurs quickly at room temperature. What might be predicted for the three thermodynamic quantities for the reverse reaction?
A) Only ΔS would be positive
B) Only ΔS would be negative
C) Only ΔG would be positive
D) All 3 would be negative
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.
A 13 gram sample of an unknown hydrocarbon is a gas at room temperature (25 °C) and atmospheric pressure. Its volume is 11.2 liters. Which of the following could the hydrocarbon be? Note: the ideal gas constant is 0.082 (Latm)/(Kmol)
A) CH
B) C2H4
C) C2H2
D) C3H3
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 = (111.2)/(0.082298). 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.
Which of the following should be observed with the addition of a catalyst to a chemical reaction?
A) The formation of a new product
B) The consumption of the catalyst
C) The activation energy is lowered
D) The decrease in enthalpy
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].
A chemist has flask containing a mixture of two gases at STP. The average velocity of a Gas 1 molecule is twice that of a Gas 2 molecule. Among the following, Gases 1 and 2 are most likely:
He = 4 g/mol Ne = 20 g/mol Ar = 40 g/mol Kr = 80 g/mol
A) He and Ar
B) He and Kr
C) Ne and Ar
D) Ne and Kr
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).
What is a possible electronic structure for an excited Cu(I) ion?
A) [Ar] 4s1 3d10
B) [Ar] 3d10
C) [Ar] 4s2 3d9
D) [Ar] 3d9 4p1
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.
The Ksp of Ag3PO4 is approximately 2 x 10^-18. Which of the following would increase the Ksp?
A) Add a solution of Na3PO4
B) Raise the pH
C) Lower the pH
D) None of the above
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.
Which of the following statements do not apply to ideal gas behavior?
I. Gas molecules move more rapidly at higher temperatures
II. It is difficult to compress gases beyond certain pressures, owing to intermolecular repulsions
III. Gases liquify as they are cooled beyond a certain point
A) I only
B) II only
C) I and III only
D) II and III only
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.
A liquid that has stronger cohesive forces than adhesive forces would have which type of meniscus?
A) Flat
B) Concave
C) Convex
D) Parabolic
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.
How many liters of 0.3M HCl can be made from a 2L stock solution of 1.5M HCl?
A) 10L
B) 12L
C) 6L
D) 20L
A
```
M1V1 = M2V2
0.3)(V1) = (1.5)(2
V1 = 10 L
~~~
As you move from left to right across a Period in the Periodic Table, which trend of elementary properties does NOT occur?
A) Metallic properties decrease
B) Atomic radius increases
C) Electronegativity increases
D) First ionization energy increases
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.
At 25C, a one liter sample of pure water has a vapor pressure of 18.5 torr. If 10 g of NaCl are added to the sample, the vapor pressure of the water will:
A) Decrease
B) Increase
C) Remain unchanged
D) Be equal to he vapor pressure of the added NaCl
A
The addition of a solute to a liquid always lowers the vapor pressure of the liquid.
Which of the following anions is the strongest Bronsted base?
A) CH3COO-
B) NO3-
C) HSO4-
D) H2PO4-
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.
Describe the reaction below.
PCl3(g) + Cl2(g) -> PCl5(g); Delta(Hf) = -87.9kJ/mol
A) Spontaneous at all temperatures
B) Nonspontaneous at all temperatures
C) ΔGrxn < 0 at only low temperatures
D) Spontaneous at only high temperatures
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.
An amino acid in a medium with a pH value above its isoelectric point would have:
A) A neutral charge
B) A net positive charge
C) A net negative charge
D) Either a net positive or net negative charge depending on the pKa value
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.
According to the second law of thermodynamics, which of the following reactions would be favored?
A) 2H2O + O2 → 2H2O2
B) ADP + Pi → ATP
C) 2H2O2 → 2H2O + O2
D) 2Na + Cl2 → 2NaCl
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.
When a chemical bond is between a strongly positive and a strongly negative element, the bond is called what?
A) A covalent bond
B) An ionic bond
C) A hydrogen bond
D) A polar covalent bond
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.
2 Ag2O -> 4 Ag + O2
A chemist runs the above reaction and produces one mole of Ag. How many grams of oxygen gas are produced?
A) 2 g
B) 4 g
C) 8 g
D) 16 g
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.
Which opposite Phase Changes occur at the same temperature for a pure substance?
A) Melting and sublimation
B) Evaporation and boiling
C) Condensation and sublimation
D) Boiling and condensation
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.
For which of the gas-phase reactions below would delta S be expected to be negative?
I. 2H2O –> 2H2 + O2
II. 2H –> H2
III. Cl2 + PCl3 –> PCl5
A)I only
B) II only
C) I and II only
D) II and III only
D
In each of II and II, the number of moles of gaseous products is less than that of gaseous reactants; thus “disorder” decreases.
Which of the following is the electron configuration of Ca2+?
A) [Ar]
B) [Ar] 4s2
C) [Ar] 4s2 3d2
D) [Ar] 4s2 4p2
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.
For a very weak acid, you would expect the pH to be:
A) Higher than the pKa
B) Equal to the pKa
C) Lower than the pKa
D) Equal to double the pKa
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.
Photons and other energy sources can excite electrons into higher energy levels, though these excitations are typically short-lived. X-ray imaging succeeds because different tissue types are differently able to absorb the energy of a photon in order to excite an electron. Suppose an electron falls from n = 4 to its ground state, n = 1. Which of the following effects is most likely?
A) A photon is absorbed
B) A photon is emitted
C) The electron moves into a p-orbital
D) The electron moves into a d-orbital
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.
The density of an unknown gas under STP conditions is 3.17 g.L-1. What is the molecular mass of the gas?
A) 3.17/24.5 = 0.13
B) 3.17/22.4 = 0.14
C) 3.17 x 24.5 = 77.7
D) 3.17 x 22.4 = 71.0
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.
Which of the following is NOT needed to calculate the endpoint of a titration?
A) Initial volume of titrant
B) Final volume of titrant
C) Concentration of the titrant
D) pH of the titrant
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.
The following statement is a description of which Gas Law? “As a gas is heated under constant pressure its volume will increase in proportion to the change in temperature.”
A) Dalton’s law of Partial Pressures
B) Boyle’s gas law
C) Charles’s gas law
D) The Ideal gas law
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
The rate at which a chemical reaction occurs is dependent on many factors. Which of the following will increase the rate of reaction?
A) Decreasing the temperature
B) Adding a catalyst
C) Decreasing the concentration of a reactant in solution
D) Decreasing the partial pressure of a reactant gas
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.
People who have high blood pressure often go on low sodium diets, in which they substitute salt (NaCl) for another compound. Which of the following would be an appropriate supplementary salt?
A) MgCl2
B) KCl
C) BeCl2
D) SrCl2
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.
For the Henderson-Hasselbalch equation, under what conditions would you expect the pKa of the acid to equal the pH?
A) When the acid has not dissociated at all
B) When the amount of conjugate base is the same as the acid
C) When the acid is completely dissociated
D) When the acid is a weak acid
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.
Which of the following is true?
A) The conjugate base of a strong acid is a stronger base
B) As pKa increases, acidity decreases
C) The conjugate acid of a weak base is a very weak acid
D) As pKa increases, acidity increases
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.
The value of the water dissociation constant Kw varies with temperature. Its value is normally given as 1.00 x 10-14 mol2 dm-6 at room temperature but 1.00 x 10-13 mol2 dm-6 at 60 °C. What is the pH of pure water at 60 °C?
A) Equal to 7.0 thus the water is neutral
B) Less than 7.0 thus the water is acidic
C) Less that 7.0 but the water is basic
D) Less that 7.0 but the water is neutral
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.
An aqueous solution of a metal nitrate (“X”NO3) forms a precipitate when magnesium sulfate (MgSO4) is added. However, a separate sample of the metal nitrate does not form any precipitate when mixed with hydrogen perchlorate (HClO4). What could this metal “X” be?
A) Sodium (Na) B) Zinc (Zn) C) Lead (Pb) D) Sulfur (S) E) Potassium (K)
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.
A sealed vessel holds air and liquid water. Which of the following conditions would greatly increase the amount of air dissolved into the water?
A) Increasing the size of the vessel
B) Decreasing the temperature of the container
C) Adding argon gas to the vessel
D) Increasing the temperature of the container
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.
Which of the following has the greatest ionic character in its bonds?
A) RbCl
B) LiCl
C) BCl3
D) CaCl2
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.
A catalyst would change which of the following?
A) Enthalpy
B) Activation energy
C) Entropy
D) Change of Enthalpy
B
A catalyst lowers the activation energy, Ea, of a reaction.
Calculate the concentration of F- ions in a 2 M solution of hydrogen fluoride, HF. The Ka of HF is 6.8 x 10^-4.
A) 1.4 x 10^-3 M
B) 1.2 x 10^-2 M
C) 1.8 x 10^-2 M
D) 3.7 x 10^-2 M
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.
Which of the following are true regarding ideal gas assumptions?
I. Gas molecules have no volume.
II. Pressure is caused by collisions against the walls of the container.
III. All collisions between molecules are elastic.
A) I only
B) I and II only
C) I and III only
D) I, II, and III
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.
2.0 moles of H2O vapor at 0.20 atm and 298K are allowed to expand so that volume doubles. The temperature is unchanged. What is the new pressure (in atm)?
A) 0.05
B) 0.1
C) 0.2
D) 0.4
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.
Hydrogen gas generated in a reaction is collected over water at 25 degrees Celsius and is found to have a pressure of 746 torr. The vapor pressure of water at 25 degrees Celsius is 24 torr. Which expression can be used to find the pressure of H2, in atm?
A) 24/760
B) 746/760
C) (746 + 24)/760
D) (746 – 24)/760
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.
A sample of H2 gas occupies .20 L at 100 degrees Celsius and 760 mm of Hg pressure. To find the number of moles of gas in the sample, a student should calculate which of the following?
A) (0.082)(100)/(760)(.20)
B) (0.082)(100)/(1.00)(.20)
C) (1.00)(0.20)/(0.082)(100)
D) (1.00)(0.20)/(0.082)(373)
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.
Which of the following statements about van der Waals equation are true?
I. At low densities, a real gas behaves like an ideal gas
II. The “b” term in the equation describes intermolecular forces
III. The product PV equals a constant for real and ideal gases alike
A) I only
B) II only
C) I and II only
D) I and III only
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.
Which of the following compounds will not react with acetyl chloride?
A) NH3
B) CH3NH2
C) (CH3)2NH
D) (CH3)4N+Cl-
D
Quaternary nitrogen atoms cannot react to form an amide because it already has four bonds.
Which of the following will NOT change the pressure in a closed vessel containing N2 gas?
I. Increasing the temperature
II. Replacing half the N2 molecules with helium atoms
III. Doubling the volume of the vessel
A) I only
B) II only
C) II and III only
D) I and II only
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.
What is the maximum number of electrons in an atom that can have n = 3, l = 2 quantum numbers?
A) 5
B) 8
C) 10
D) 15
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).
Magnetic properties are due to repulsion. Which of the following ground state ions is/are diamagnetic?
This question requires the use of a periodic table.
I. Fe2+ II. Zn2+ III. Cu1+ IV. Ni2+ V. V2+
A) I and IV only
B) I, IV, and V only
C) II only
D) II and III only
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.
For those atoms in the first column of the periodic table which form stable positive ions, the ion usually has a considerably smaller radius than the neutral atom. What explains this effect?
A) The outer electron(s) effectively screen the nuclear charge
B) The ion has a greater nuclear charge
C) The neutral atom has a greater nuclear charge
D) The neutral atom has an outer electron in an orbital of higher “n” than the ion
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.
Which of the following has (have) buffer properties?
I. 50 mL of .1 M HCl
II. 100 mL of .1 M acetic acid
III. 50mL of .1 M NaOH
A) I only
B) II only
C) III only
D) The solution resulting when II and III are mixed
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.
According to scientists, Oparin and Haldane, which of the following gases were present in the primitive atmosphere?
A) CH4, NH3, H2, H2O
B) CO2, NH3, H2, H2O
C) N2, O2, H2, H2O
D) CH4, NH3, O2, H2O
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.
Which of the following number sets is possible?
A) n = 2; l = 2; mt = 1; ms = +1/2
B) n = 2; l = 1; mt = -1; ms = +1/2
C) n = 2; l = 0; mt = -1; ms = -1/2
D) n = 2; l = 0; mt = 1; ms = -1/2
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.
What is the maximum number of electrons allowed in a single atomic energy level in terms of the principal quantum number n?
A) 2n
B) 2n + 2
C) 2n^2
D) 2l^2 + 2
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.
Which of the following equations describes maximum number of electrons that can fill a subshell?
A) 2l + 2
B) 4l + 2
C) 2l^2
D) 2l^2 + 2
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
Which of the following atoms only has paired electrons in its ground state?
A) Sodium
B) Iron
C) Cobalt
D) Helium
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.
