AAMC 2 - Chem/Phys

Question 1

True or False:

A peptide bond is a covalent bond

Answer 1

True.

Question 2

What amino acid is frequently located at the active site of metalloenzymes, and what structure is part of its side chain?

Answer 2

Histidine is an amino acid whose side chain is frequently encountered at the active site of metalloenzymes.

The side chain of histidine contains the five-membered imidazole ring.

Question 3

The concentration of enzyme for each experiment was 5.0 μM. What is k_cat for the reaction at pH 4.5 with NO chloride added when Compound 3 is the substrate?

A. 2.5 × 10^–2 s^–1

B. 1.3 × 10² s^–1

C. 5.3 × 10³ s^–1

D. 7.0 × 10⁵ s^–1

Answer 3

A. 2.5 × 10^–2 s^–1

The fact that the rate of product formation did not vary over time for the first 5 minutes implies that the enzyme was saturated with substrate. Under these conditions, k_cat=V_max/[E]

= (125 nM/s)/5.0 μM

= 2.5 × 10^–2s^–1

Question 4

What is k_cat?

Answer 4

K_cat =turnover of substrate to product / unit time

k_cat measures the number of substrate molecules “turned over,” or converted to product, per enzyme molecule per second.

Question 5

What is the equation for V_max?

Answer 5

V_max = [E]*k_cat

_{E is the enzyme concentration}

Question 6

What equation is used to identify the efficiency of an enzyme?

Answer 6

k_cat/K_m

The ratio of k_cat/K_m is referred to as the catalytic efficiency of the enzyme. A large kcat (high turnover) or a small Km (high substrate affinity) will result in a higher catalytic efficiency, which indicates a more efficient enzyme.

Question 7

What 2 changes could indicate a more efficient enzyme?

Answer 7

A large k_cat (high turnover) or a small K_m (high substrate affinity) will result in a higher catalytic efficiency, which indicates a more efficient enzyme.

The ratio of k_cat/K_m is referred to as the catalytic efficiency of the enzyme.

Question 8

Absorption of ultraviolet light by organic molecules always results in what process?

A. Bond breaking

B. Excitation of bound electrons

C. Vibration of atoms in polar bonds

D. Ejection of bound electrons

Answer 8

B. Excitation of bound electrons

The absorption of ultraviolet light by organic molecules always results in electronic excitation. Bond breaking can subsequently result, as can ionization or bond vibration, but none of these processes are guaranteed to result from the absorption of ultraviolet light.

Question 9

The absorption of ultraviolet light by organic molecules ALWAYS results in __________.

Answer 9

The absorption of ultraviolet light by organic molecules ALWAYS results in electronic excitation.

Question 10

Four organic compounds: 2-butanone, n-pentane, propanoic acid, and n-butanol, present as a mixture, are separated by column chromatography using silica gel with benzene as the eluent. What is the expected order of elution of these four organic compounds from first to last?

A. n-Pentane → 2-butanone → n-butanol → propanoic acid

B. n-Pentane → n-butanol → 2-butanone → propanoic acid

C. Propanoic acid → n-butanol → 2-butanone → n-pentane

D. Propanoic acid → 2-butanone → n-butanol → n-pentane

Answer 10

A. n-Pentane → 2-butanone → n-butanol → propanoic acid

The four compounds have comparable molecular weights, so the order of elution will depend on the polarity of the molecule. Since silica gel serves as the stationary phase for the experiment, increasing the polarity of the eluting molecule will increase its affinity for the stationary phase and increase the elution time (decreased R_f).

Question 11

When using the equation E=hc/wavelength,

What does the variable of wavelength refer to?

Answer 11

The wavelength of excitation

Question 12

Define the half-life of a radioactive material

Answer 12

The half-life of a radioactive material is defined as the time it takes for half of all the radioactive nuclei to decay into their daughter nuclei, which may or may not also be radioactive.

Question 13

Are ketones more polar than alcohols? Why/why not?

Answer 13

No.

An alcohol is more polar than a ketone due to its ability to hydrogen bond. Since ketones lack an OH, they are incapable of intermolecular hydrogen bonding.

Question 14

A person is sitting on a chair as shown.

Why must the person either lean forward or slide their feet under the chair in order to stand up?

A. To increase the force required to stand up

B. To use the friction with the ground

C. To reduce the energy required to stand up

D. To keep the body in equilibrium while rising

Answer 14

D. To keep the body in equilibrium while rising

The answer to this question is D because as the person is attempting to stand, the only support comes from the feet on the ground. The person is in equilibrium only when the center of mass is directly above their feet. Otherwise, if the person did not lean forward or slide the feet under the chair, the person would fall backward due to the large torque created by the combination of the weight of the body (applied at the person’s center of mass) and the distance along the horizontal between the center of mass and the support point.

Question 15

What is the net charge of sT-loop (KTFCGPEYLA) at pH 7.2?

A. –2

B. –1

C. 0

D. +1

Answer 15

C. 0

The answer to this question is C because at pH 7.2, the N-terminus will be positively charged and the C-terminus will be negatively charged. In addition, the lysine side chain will carry one positive charge and the glutamic acid side chain will carry one negative charge.

Question 16

Describe the relationship between Hill’s Coefficient and cooperativity.

Answer 16

If the Hill’s Coefficient is greater than 1, positive cooperative binding is occuring. The more bound, the greater affinity to bind.

If the Hill’s Coefficient is less than 1, negative cooperative binding is occuring. The more bound, the worse affinity to bind.

If the Hill’s Coefficient is equal to 1, no cooperativity is present.

Question 17

In designing an experiment measuring the activity of kinases, the researchers should use which type of ³²P labeled ATP?

A. α³²P-ATP

B. β³²P-ATP

C. γ³²P-ATP

D. δ³²P-ATP

Answer 17

C. γ³²P-ATP

The answer to this question is C because the phosphoryl transfer from kinases comes from the γ-phosphate of ATP.

Question 18

A patient puts on a mask with lateral openings and inhales oxygen from a tank as shown.

What phenomenon causes static air to be drawn into the mask when oxygen flows?

A. Doppler effect

B. Venturi effect

C. Diffusion

D. Dispersion

Answer 18

B. Venturi effect

The answer to this question is B because oxygen pressure is the sum of the oxygen static pressure P and the oxygen flow pressure ρv²/2. In the area of the mask openings, P_air = P + ρv²/2, thus P_air > P. Air enters the mask because the static pressure of the air is larger than the static pressure of the oxygen in flow. This is the Venturi effect, and the mask is called the Venturi mask.

Question 19

What causes duplex DNA with a certain (A + T):(G + C) ratio to melt at a higher temperature than comparable length duplex DNA with a greater (A + T):(G + C) ratio?

A. Stronger van der Waals forces of pyrimidines

B. Stronger van der Waals forces of purines

C. Increased π- stacking strength

D. Reduced electrostatic repulsion of phosphates

Answer 19

C. Increased π- stacking strength

G–C base pairs form stronger π-stacking interactions than A–T base pairs, thereby creating the most thermal stability.

Jack Westin Advanced Solution:

While this is not explicitly talking about the additional hydrogen bonds between G+C, that extra hydrogen bond does mean less rotation and strain, and this increases π- stacking strength.

Question 20

If a double helical strand of DNA has twice as many G-C pairs than another strand, how many more purines does it have?

Answer 20

All double-stranded DNA strands of the same length contain the exact same number of purines and pyrimidines.

Question 21

Which property of a substance is best used to estimate its relative vapor pressure?

A. Melting point

B. Boiling point

C. Molecular weight

D. Dipole moment

Answer 21

B. Boiling point

The boiling point of a liquid is the point at which vapor pressure above the liquid equals atmospheric pressure.

Question 22

What are the structural features possessed by storage lipids?

A. Two fatty acids ester-linked to a single glycerol plus a charged head group

B. Three fatty acids ester-linked to a single glycerol

C. Two fatty acids ester-linked to a single sphingosine plus a charged head group

D. Three fatty acids ester-linked to a single sphingosine

Answer 22

B. Three fatty acids ester-linked to a single glycerol

The answer to this question is B because triacylglycerols are neutral storage lipids. They consist of three fatty acids ester-linked to a single glycerol.

Question 23

What type of biomolecule is being referred to by this description?

Two fatty acids ester-linked to a single glycerol plus a charged head group

Answer 23

This describes a phospholipid.

Phospholipids consist of a glycerol molecule, two fatty acids, and a phosphate group that is modified by an alcohol. The phosphate group is the negatively-charged polar head.

Question 24

Define Boyle’s Law

Answer 24

Boyle’s law states that the absolute pressure and volume of a given mass of confined gas are inversely proportional, provided the temperature remains unchanged within a closed system.

Question 25

_______ law states that the absolute pressure and volume of a given mass of confined gas are inversely proportional, provided the temperature remains unchanged within a closed system.

Answer 25

Boyle’s law states that the absolute pressure and volume of a given mass of confined gas are inversely proportional, provided the temperature remains unchanged within a closed system.

Question 26

Define Charle’s Law

Answer 26

Charles’s law describes the relationship between the volume and temperature of a gas. This law states that at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin); in other words, temperature and volume are directly proportional.

Question 27

_______ law describes the relationship between the volume and temperature of a gas. This law states that at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin); in other words, temperature and volume are directly proportional.

Answer 27

Charles’s law describes the relationship between the volume and temperature of a gas. This law states that at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin); in other words, temperature and volume are directly proportional.

Question 28

The lone pair of electrons in ammonia allows the molecule to:

A. assume a planar structure.

B. act as an oxidizing agent.

C. act as a Lewis acid in water.

D. act as a Lewis base in water.

Answer 28

D. act as a Lewis base in water

By definition, a Lewis base is a substance that donates an electron pair in forming a covalent interaction.

Question 29

Define a Lewis Acid

Answer 29

A Lewis acid is any substance that can accept a pair of electrons.

Question 30

What is the role of the solid-state catalyst in the Haber process?

A. It increases the amount of ammonia produced per unit time.

B. It increases the total amount of ammonia produced.

C. It decreases the amount of ammonia that decomposes per unit time.

D. It decreases the total amount of ammonia produced.

Answer 30

A. It increases the amount of ammonia produced per unit time.

Where do we see catalysts the most? Enzymes! In this specific question, we have to know what a catalyst is, and why it is used in the Haber process. Enzymes are catalysts that are able to lower activation energy without being used up or being destroyed in the reaction. Catalysts work to increase the rate of a reaction by lowering activation energy.

Question 31

How can you determine the spontaneity of a reaction when given a K constant and temperature?

Answer 31

We can use the equation ΔG°=-RTlnK.

If we have a positive T and a positive K, that means we have a negative ΔG° and a spontaneous reaction.

Question 32

Describe a coordinate covalent bond

Answer 32

In a coordinate covalent bond, both of the shared electrons originated on the same atom. Generally, this means that a lone pair of one atom attacked another atom with an unhybridized p-orbital to form a bond. Once such a bond forms, however, it is indistinguishable from any other covalent bond.

Question 33

In [Cu(NH₃)₄]²⁺, the subscript 4 indicates which of the following?

A. The oxidation number of Cu only

B. The coordination number of Cu²⁺ only

C. Both the oxidation number of Cu and the coordination number of Cu²⁺

D. Neither the oxidation number of Cu nor the coordination number of Cu²⁺

Answer 33

B. The coordination number of Cu²⁺ only

The coordination number is the number of atoms, molecules, or ions bonded to a central atom in a molecule. In this case, the copper is bonded to the four ammonia.

Because ammonia is neutral, the number 4 reflects only the number of ammonia molecules that bind to the central Cu²⁺ cation and does not indicate anything about its oxidation number.

Question 34

The __________ is the number of atoms, molecules, or ions bonded to a central atom in a molecule.

Answer 34

The coordination number is the number of atoms, molecules, or ions bonded to a central atom in a molecule.

Question 35

Which of the following atoms will be expected to have the smallest second ionization energy?

A. Na

B. C

C. O

D. Ca

Answer 35

D. Ca

Calcium is an alkaline earth metal which means it has a small second ionization energy. After losing the first electron, calcium would only have a single valence electron remaining, so it would be more willing to lose this remaining electron to achieve a valence octet.

Question 36

Which of the following species has the largest mass percent of oxygen?

A. H₂O

B. CaCO₃

C. CO₂

D. HCO₃^–

Answer 36

A. H₂O

The answer to this question is A because mass percent of oxygen is calculated by multiplying the coefficient for oxygen in the formula of the substance by 16 and then dividing by the molar mass of the substance. Water has the highest percentage of oxygen by mass of the compounds listed (16/18) × 100 = 89%.

Question 37

What is the pH of a buffer solution that is 0.2 M in HCO₃^– and 2 M in H₂CO₃? (Note: The first pK_a of carbonic acid is 6.37.)

A. 4.37

B. 5.37

C. 6.37

D. 7.37

Answer 37

B. 5.37

The pH of the solution can be calculated using the Henderson-Hasselbach equation:

pH = pK_a + log([base]/[acid])

Plugging in the values provided in the question gives pH = 6.37 + log(0.2/2)

log(0.2/2) = log(0.1) = -1

pH = 6.37 + (-1)

= 5.37

Question 38

What is the concentration of Ca²⁺(aq) in a saturated solution of CaCO₃? (Note: The solubility product constant K_sp for CaCO₃ is 4.9 × 10^–9.)

A. 2.4 × 10^–4 M

B. 4.9 × 10^–5 M

C. 7.0 × 10^–5 M

D. 4.9 × 10^–9 M

Answer 38

C. 7.0 × 10^–5 M

The solubility product constant expression for CaCO₃ is K_sp= [Ca²⁺][CO₃^2–].

Since equal quantities of Ca²⁺(aq) and CO₃^2–(aq) are produced when CaCO₃ dissolves, this expression reduces to 4.9 × 10^–9=x²,or49 × 10^–10 = x². This can be solved directly by taking the square root of each side.

Question 39

An inflatable cuff was used to temporarily stop blood flow in an upper arm artery. While releasing the pressure to deflate the cuff, a stethoscope was used to listen to blood flow in the forearm. The blood pressure reading was 130/85. Given this information, which of the following statements is LEAST likely to be true?

A. 85 mmHg was the diastolic pressure.

B. Blood flow was heard when the pressure of the cuff was greater than 130 mmHg.

C. 130 mmHg was the systolic pressure.

D. Blood flow was heard when the pressure of the cuff was 90 mmHg.

Answer 39

B. Blood flow was heard when the pressure of the cuff was greater than 130 mmHg.

The answer to this question is B because the question indicates that the cuff was inflated to temporarily stop blood flow in the artery. The systolic pressure is determined from the first sound of blood flow that can be heard once the pressure exerted by the inflatable cuff falls below the pressure in the artery. The blood pressure reading was 130/85, which indicates that blood flow started again when the pressure was 130 mmHg. Therefore, blood flow was not heard when the pressure of the cuff was greater than 130 mmHg.

A is not the correct response because the lower number of the blood pressure reading represents the diastolic pressure. C is not the correct response because the higher number of the blood pressure reading represents the systolic pressure. D is not the correct response because blood flow would be heard when the pressure of the cuff is 90 mmHg, as this pressure is higher than the diastolic pressure.

Question 40

Which statement correctly describes the structure of the DNA double helix?

A. Nitrogenous bases pair with other bases in the same purine or pyrimidine groups.

B. The two DNA strands of the double helix are oriented in the same direction.

C. The amount of guanine will equal the amount of cytosine in a DNA sequence.

D. Sugar-phosphate backbones form the interior of the double helix.

Answer 40

C. The amount of guanine will equal the amount of cytosine in a DNA sequence.

The answer to this question is C because as guanine and cytosine form base pairs on opposite DNA strands, they will occur in equal amounts within a specific DNA sequence.

A is incorrect because purines and pyrimidines do not form base pairs with other members within the same group. B is incorrect because the strands are antiparallel, meaning that they are oriented in opposite directions. D is incorrect because sugar-phosphate backbones form the exterior of the double helix.

Question 41

The three ions in the following figure are initially held fixed as shown.

If Ion C alone were released, in which of the following directions would it begin to move?

A. Toward the top

B. Toward the bottom

C. Toward the left

D. Toward the right

Answer 41

D. Toward the right

The answer to this question is D because the released ion moves under the simultaneous effect of two forces: from ion A, toward the right and the top, and from ion B, toward the bottom and to the right. Because the forces depend inversely proportionally on the distances between the ions and directly proportionally on the product between their charges (which are all the same in absolute value), the resultant force is to the right because the top and bottom components cancel out.

Question 42

A glass rod is rubbed with a silk scarf producing a charge of +3.2 × 10^–9 C on the rod. (Recall that the magnitude of the proton and electron charges is 1.6 × 10^–19 C.) The glass rod has:

A. 5.1 × 10¹¹ protons added to it.

B. 5.1 × 10¹¹ electrons removed from it.

C. 2.0 × 10¹⁰ protons added to it.

D. 2.0 × 10¹⁰ electrons removed from it.

Answer 42

D. 2.0 × 10¹⁰ electrons removed from it.

The answer to this question is D because the number of charges in excess can be computed as

+3.2 × 10 ^–9 C/1.6 × 10 ^–19 C = +2.0 × 10 ¹⁰

This means that the rod has an excess of positive charge, created by removing a number of +2.0 × 10 ¹⁰ electrons from the material, as it is not possible to add protons in a manner described in this question.

Question 43

Which single bond present in nitroglycerin (can be solved without seeing this) is most likely the shortest?

A. C–H

B. C–O

C. C–C

D. O–N

Answer 43

A. C–H

The answer to this question is A. All of the bonds listed are single bonds. Since hydrogen has a much smaller atomic radius than second period elements, the covalent bond between C and H is shorter than any of the other bonds listed.

Question 44

What is the coordination number?

Answer 44

The coordination number is the number of atoms that surround and are bonded to a central atom.

The coordination number is the relevant factor in determining molecular geometry

Question 45

What is the molar volume of an ideal gas at STP?

Answer 45

The molar volume of an ideal gas at STP is 22.4 L/mol.

Question 46

How much work did an 83-year-old female do while stretching the rubber band to the limit of her strength? (she stretched the band 20cm, which has a force constant of 200 N/m)

A. 4 J

B. 5 J

C. 6 J

D. 7 J

Answer 46

A. 4 J

Now we’re dealing with stretching a rubber band, so we’ll consider potential energy. Work done on an elastic object can be given by:

Work = ½ kx²

Work = ½ (200 N/m) (0.2)² = 4.0 J

Question 47

Which of the following energy conversions best describes what takes place in a battery-powered resistive circuit when the current is flowing?

A. Electric to thermal to chemical

B. Chemical to thermal to electric

C. Electric to chemical to thermal

D. Chemical to electric to thermal

Answer 47

D. Chemical to electric to thermal

AAMC: The answer to this question is D because the chemical energy of the battery elements is used as electrical energy to set the charge carriers in motion through the resistor, where they experience drag from the crystal lattice of the resistive conductor and dissipate their energy as heat from the resistor.

Jack Westin: This is going to come down to a few steps and knowing our content. A redox reaction is taking place in a battery (which is chemical energy), but is converted to electric current, while current is running, heat is going to dissipate as we have resistance (thermal). We can pick answer choice D as our best answer here. Unfortunately, there aren’t many ways around knowing how to solve this problem besides committing the content to memory. I highly recommend reviewing our content outline online if you are not comfortable with circuit elements specifically.

Question 48

Protein secondary structure is characterized by the pattern of hydrogen bonds between:

A. backbone amide protons and carbonyl oxygens.

B. backbone amide protons and side chain carbonyl oxygens.

C. side chain hydroxyl groups and backbone carbonyl oxygens.

D. side chain amide protons and backbone carbonyl oxygens.

Answer 48

A. backbone amide protons and carbonyl oxygens.

The answer to this question is A because secondary structure is represented by repeated patterns of hydrogen bonds between the backbone amide protons and carbonyl oxygen atoms.

Question 49

A 60-Ω resistor is connected in parallel with a 20-Ω resistor. What is the equivalent resistance of the combination?

A. 80 Ω

B. 40 Ω

C. 15 Ω

D. 3 Ω

Answer 49

C. 15 Ω

The answer to this question is C because the equivalent resistance is given by the expression

(1/(60) + 1/(20))^–1 = 15

Question 50

What is the name of the following amine?

Answer 50

Imidazole

Imidazole has a 5-membered, aromatic ring structure.

Question 51

What is a toluene?

Answer 51

A toluene is a benzene ring bound to a methyl group

Question 52

Gap junctions are composed of which proteins?

Answer 52

Gap junctions are composed of connexins. In gap junctions, these proteins are arranged around a small hole or pore through which small solutes can diffuse.

Question 53

Microfilaments and microtubules both undergo a process called treadmilling. Treadmilling is best described as:

Answer 53

The simultaneous addition of subunits to one end of a strand and loss of subunits from the opposite end

Subunits are lost from one end of a filament at the same time that more subunits add to the other end, which means that one end is shrinking as the other grows. Visually, this gives the impression of one section of a filament moving steadily across the cytoplasm. This is treadmilling.

Question 54

Which of these molecules would display little to no observable activity on an IR spectrum?

A. Carbon dioxide

B. Methane

C. Nitrogen

D. Nitric oxide

Answer 54

C. Nitrogen

To be IR-active, a molecule must have an inducible dipole moment. (While a permanent dipole is not necessary, it also confers IR activity.) N2, a nonpolar diatomic molecule, has no dipole moment; electron density is shared equally between the two nitrogen atoms. When excited by infrared light, the molecule is only induced to stretch along its axis, which does not change its overall dipole moment. It exhibits no bending or asymmetric stretching vibrational modes.

Question 55

When is the only time that oxygen will have an oxidation state of +2?

Answer 55

When bound to fluorine

Question 56

When is the only time that oxygen will have an oxidation state of -1?

Answer 56

When in a peroxide

Question 57

Perchloric acid is a strong acid with a pK_a of -1.58. Two moles of perchloric acid is reacted with 4 L of a 0.5 M solution of NaOH, resulting in water and Species A. The oxidation state of oxygen in Species A is:

A. -2

B. -1

C. 0

D. +2

Answer 57

A. -2

Perchloric acid is HClO₄, meaning that Species A must be NaClO₄. In this compound, as in most cases other than peroxides, oxygen has an oxidation state of -2. Sodium has an oxidation number of +1, while chlorine has one of +7 and the overall compound is neutral.

Question 58

Impurities in a sample of TLC would have what result?

Answer 58

A smear instead of a distinct spot for that impure compound.

Question 59

Which fundamental law of physics prohibits an engine from having an efficiency that is greater than 100%?

A. The First Law of Thermodynamics

B. The Second Law of Thermodynamics

C. The Theory of Special Relativity

D. No law; this is just a current obstacle related to modern technology.

Answer 59

A. The First Law of Thermodynamics

It is the first law that states that energy cannot be created or destroyed, only converted between forms. In other words, we cannot possibly obtain a larger energy output than was originally supplied. Such a scenario would be required to yield an efficiency of greater than 100%.

Question 60

Which is more acidic, the proton on an –OH group or that present on an –SH group?

A. The proton on the –OH group, because oxygen is more electronegative than sulfur

B. The proton on the –OH group, because oxygen is smaller than sulfur

C. The proton on the –SH group, because sulfur is less electronegative than oxygen

D. The proton on the –SH group, because sulfur is larger than oxygen

Answer 60

D. The proton on the –SH group, because sulfur is larger than oxygen

Since sulfur is much larger than oxygen, the conjugate base of a thiol-containing molecule is better able to delocalize negative charge, increasing its stability. Remember, the more stable the conjugate base, the stronger the acid.

A is wrong because while electronegativity does promote acidity, this trend exists across a period, not down a group. Here, atomic radius has a more substantial effect.

Question 61

Elemental nitrogen does not exhibit an IR spectrum because it:

A. has two lone pairs of electrons.

B. does not vibrate.

C. is symmetrical.

D. exists in a gaseous state.

Answer 61

C. is symmetrical.

While infrared spectroscopy is an enormously complex technique, it basically measures the way in which the dipole moment of a bond vibrates over time. Since a diatomic molecule that is composed of two identical atoms (like N₂) does not have a dipole, it gives no signal.

Question 62

While carbon dioxide exhibits four vibrational modes in space, its experimentally-obtained IR spectrum shows no more than two distinct peaks. This is partly because:

A. the two oxygen nuclei exhibit nuclear spins that are equal in magnitude but opposite in direction.

B. three of the vibrational modes are considered energetically degenerate.

C. one of the vibrational modes is not observable by IR spectroscopy.

D. two of the vibrational modes are not excitable by IR spectrometers of conventional power.

Answer 62

C. one of the vibrational modes is not observable by IR spectroscopy.

The symmetric stretching mode of this molecule does not induce a dipole. It is thus effectively invisible to IR spectroscopy.

Question 63

What are the units of k in a 0th order reaction?

Answer 63

M*s^-1

or

M/s

Question 64

What are the units of k for a 1st order reaction?

Answer 64

s^-1

or

1/s

Question 65

What are the units of k for a second order reaction?

Answer 65

M^-1*s^-1

or

1/(M+s)

Question 66

What are the units of k for a 3rd order reaction?

Answer 66

M^-2*s^-1

or

1/(M²+s)

Question 67

True or false:

The order of a reaction can be determined from the stoichiometry of that reaction.

Answer 67

This statement is false.

The order of a reaction must be determined experimentally (e.g. by a passage telling you the amount of a reactant and its resultant reaction rate). On the MCAT™, do not assume the coefficients in a reaction will tell you the rate.

Question 68

In anion-exchange chromatography, what type of ions will be attracted to the stationary phase?

Answer 68

In anion-exchange chromatography, the stationary phase attracts negatively-charged ions.

Question 69

True or False:

An electrophoresis apparatus has a positively-charged anode and a negatively-charged cathode.

Answer 69

This statement is true.

An electrophoresis apparatus is an example of an electrolytic cell, of which the anode is positively charged and the cathode is negatively charged.

Question 70

True or False:

An electrophoresis apparatus makes use of an applied current to drive a non-spontaneous reaction.

Answer 70

True.

An electrophoresis apparatus is an example of an electrolytic cell, wherein a current is applied to drive a non-spontaneous reaction.

Question 71

Describe the Beer-Lambert Law

Answer 71

The Beer-Lambert law gives the relationship between solute concentration and its absorption of light.

A = εlc

where A is absorptivity, ε is the molar attenuation coefficient, and c is the concentration, with l being the path length of the light. Essentially, it indicates that higher levels of absorbance mean that more light is absorbed, which means that more of a substance is present.

Question 72

A ________ purification is employed when a researcher wishes to have a large amount of the desired protein, while an ________ purification is used when a researcher desires a small protein extract for qualitative analysis.

Answer 72

A preparative purification is employed when a researcher wishes to have a large amount of the desired protein, while an analytical purification is used when a researcher desires a small protein extract for qualitative analysis.

Question 73

C₅H₆O₃ (g) ↔ C₂H₆ (g) + 3 CO (g)

If the pressure of the system is increased, which of these observations are likely?

I. The concentration of CO will increase.

II. The partial pressure of C₅H₆O₃ will increase.

III. The number of molecules of C₅H₆O₃ will increase.

IV. When equilibrium is re-established, the partial pressure of each species will be equal to its prior value.

A. I only

B. IV only

C. I and II

D. II and III

Answer 73

D. II and III

When pressure is increased, the reaction will shift in a way that minimizes the total number of gas molecules in the system. In the given reaction, this shift will be toward the reactant side to counteract the increase in pressure.

Question 74

A student needs to increase the calcium ion concentration in a particular solution. He adds a large chunk of solid calcium hydroxide, but is dismayed to note that it hardly dissolves at all. To best remedy this situation, the student should:

A. add 1 M KOH

B. add 0.05 M HC₂H₃O₂.

C. add 2 M HI

D. none of the above; none of these changes would alter K_sp.

Answer 74

C. add 2 M HI

Due to Le Châtelier’s principle, basic compounds dissolve best in acidic solution. Specifically, the presence of protons neutralizes some of the free hydroxide ions, pushing the equilibrium toward dissociation. 2 M of a strong acid would certainly accomplish the student’s goal of raising the number of Ca²⁺ ions in solution.

Question 75

The structure of sucrose, a physiologically relevant disaccharide, is shown here.

A sample of sucrose is treated with hydrochloric acid. The solution is then neutralized with a weak base, and Tollens’ reagent is added to the reaction vessel. Which of the following is likely to occur next?

A. A silver mirror will form on the surface of the vessel, since glucose is a reducing sugar.

B. A silver mirror will form on the surface of the vessel, since sucrose is a reducing sugar.

C. No silver mirror will form on the surface of the vessel, since fructose is not a reducing sugar.

D. No silver mirror will form on the surface of the vessel, since sucrose is not a reducing sugar.

Answer 75

A. A silver mirror will form on the surface of the vessel, since glucose is a reducing sugar.

Acid hydrolysis of sucrose will yield glucose and fructose, the two monomeric components of this disaccharide. Since all monosaccharides are reducing sugars, it would thus enable formation of a silver mirror, a positive result.

Question 76

A “loop-the-loop” on an amusement park ride has a radius of R. On the horizontal approach into the “loop-the-loop”, the cars travel with a velocity of v. If a man of mass M were able to step on a scale at the instant when he is at the bottom of this loop, how much would he seem to weigh?

A. Mg + Mv²/R

B. g + v²/R

C. Mv²/R

D. Mg

Answer 76

A. Mg + Mv²/R

If the man were not moving in a circular pattern, his weight would simply be Mg. Here, however, he is also subject to centripetal force.

Remember that apparent weight can be conceptualized as the normal force exerted on an object. In this case, the normal force is that which points directly upward from the bottom of the loop. This is the combination of centripetal force (Mv²/r) and gravitational force (Mg).

Question 77

Aspartic acid, lysine, and glutamine undergo cation-exchange chromatography. How should they be ordered from longest to shortest retention time?

A. Aspartic acid, lysine, glutamine

B. Glutamine, lysine, aspartic acid

C. Lysine, aspartic acid, glutamine

D. Lysine, glutamine, aspartic acid

Answer 77

D. Lysine, glutamine, aspartic acid

Cation-exchange chromatography features a column with a negatively-charged stationary phase. This phase will interact with the positive lysine molecules, while repelling aspartic acid due to its negative carboxylate side chain.

Thus, lysine will elute last, while aspartic acid will travel through the column the fastest. Glutamine, which has an uncharged polar side chain, will be neither attracted nor repelled to the stationary phase.

Question 78

The diagram below represents the electronic configuration of an arsenic atom (Z=33). It is implied that all shells except for the outermost one (which contains 4 electrons, as shown) are full.

With regard to this atom, what is the effective nuclear charge on an n = 3 electron?

A. 4

B. 10

C. 23

D. 33

Answer 78

C. 23

Effective nuclear charge can be found by subtracting the total number of electrons in all shells preceding the one in question from the nuclear charge (the number of protons).

Arsenic has an atomic number of 33. Starting from the first ring, the numbers of electrons in those shells are, respectively, 2 and 8. (Note that this information corresponds to the atom’s electronic configuration.) To calculate the effective nuclear charge when n = 3, then, we only need to find (33 – 10), or 23.

Question 79

The following table shows pK_w values for pure water with respect to temperature.

What is the pH of pure water at 75° C?

A. 6.35

B. 6.70

C. 7.00

D. 7.65

Answer 79

A. 6.35

pH + pOH = pK_w

and pH = pOH for a neutral solution.

Substituting pH in for pOH, we get 2(pH) = 12.70, or pH = 6.35.

Question 80

The table below gives the K_sp values for a number of salts in water.

Given this information, which of the following statements are true?

I. CuS is the most soluble of the three salts.

II. Mn(OH)₂ is less soluble than Mg(OH)₂.

III. Mg(OH)₂ has a higher molar solubility than CuS.

A. I only

B. II only

C. I and III

D. II and III

Answer 80

D. II and III

We see from the table that Mn(OH)₂ has a smaller K_sp (in other words, one with a more negative exponent) than Mg(OH)₂. For this reason, statement II is correct.

For statement III, remember that molar solubility is defined as the moles of a substance that can dissolve in one liter of solution. This value correlates to K_sp, and since Mg(OH)₂ and Mn(OH)₂ dissociate into the same number of ions, we can compare their K_sp values directly. As a result, we know that statement III is true as well.

Question 81

Which of the following statements is true?

I. Acetals cannot form under basic conditions.

II. Hemiacetals and hemiketals exist in equilibrium with aldehydes and ketones.

III. In multistep reactions, acetals and ketals often serve as protecting groups for vulnerable esters.

IV. Ketals contain two –OR groups, while acetals contain only one.

A. I only

B. I and II only

C. I, II, and IV only

D. I, III, and IV only

Answer 81

B. I and II only

Under basic conditions, only hemiacetals can be formed. Acetals, which result from the reaction of a hemiacetal with an alcohol, require acidic conditions (I). Hemiacetals and hemiketals rapidly interconvert into aldehydes and ketones respectively (II).

Question 82

The structure of malonic acid, an organic acid with a variety of uses in manufacturing, is shown below.

If one mole of sucrose is able to depress the freezing point of a certain volume of water by 3.9°C, what would be the freezing point of the same volume of water when one mole of malonic acid is added?

A. 0°C

B. -3.9°C

C. -7.8°C

D. -11.7°C

Answer 82

D. -11.7°C

Freezing point depression is a colligative property, meaning that it depends on the number of particles in solution. Sucrose is organic and does not dissociate in water, but carboxylic acids will lose a proton at a pH of 7. Malonic acid, then, dissociates into three particles: its deprotonated form and two free protons.

Thus, one mole will depress this volume by three times the amount that sucrose affected it, or 11.7°C. Water typically freezes at 0°C, making our new freezing point -11.7°C.

Question 83

Which period of the periodic table contains the lowest-energy electrons that may be found in a d subshell?

A. Period 3

B. Period 4

C. Period 5

D. Period 6

Answer 83

B. Period 4

The d subshell begins at period 4 and increases in energy as you move down the table. Note that the d orbitals located in period 4 are termed “3d,” not “4d,” orbitals.

Question 84

What is the difference between a “molecule” and a “compound”?

Answer 84

A molecule is any structure containing 2 or more atoms.

A compound is any structure containing 2 or more elements

For example, while O₂ is a molecule, it is not a compound because it contains oxygen alone. Methane is both a molecule and a compound.

Question 85

All of the following statements are false regarding CH₄ EXCEPT:

I. it is a molecule.

II. it is a compound.

III. two moles of it contain eight moles of hydrogen.

IV. when mixed with water, one mole yields 5 ions in solution.

A. I and III

B. I and IV

C. I, II, and III

D. I, II, III, and IV

Answer 85

C. I, II, and III

A molecule is a general term for any distinct chemical structure that contains two or more atoms.

A compound is a type of molecules that contains more than one element. For example, while O₂ is a molecule, it is not a compound because it contains oxygen alone.

Methane is both a molecule and a compound, and because it contains four moles of hydrogen in each mole of itself, two moles of methane include eight moles of hydrogen.

Question 86

Which substitution, of those below, is most likely to cause a change in the tertiary structure of a protein?

A. Val to Met

B. Lys to Leu

C. Ser to Thr

D. Asp to Glu

Answer 86

B. Lys to Leu

The tertiary structure of a protein is driven by the tendency of hydrophobic residues to bury themselves inside the molecule, away from water, as well as interactions between side chains. Specifically, a substitution involving two amino acids that vary in polarity, charge, or sulfur content could alter tertiary structure. Of all the options listed, replacing lysine (a basic, positively-charged AA) with leucine (a nonpolar, neutral AA) is most likely to promote such a change.

Question 87

What is the Michaelis-Menten equation?

Answer 87

V - reaction rate

v_max - maximum possible reaction rate

K_m - substrate concentration sufficient to reach ½ vmax

[S] - substrate concentration

Question 88

Which of the following is a true statement?

A. Both sound and ultraviolet light waves are longitudinal.

B. Both sound and ultraviolet light waves are transverse.

C. Sound is a longitudinal wave, while ultraviolet light is a transverse wave.

D. Sound is a transverse wave, while ultraviolet light is a longitudinal wave.

Answer 88

C. Sound is a longitudinal wave, while ultraviolet light is a transverse wave.

Sound is a longitudinal wave in which the medium is compressed in the same direction that the wave propagates. Light is a transverse wave involving electric and magnetic fields. Note that the fact that the question deals with UV light is irrelevant; all light on the electromagnetic spectrum consists of waves that are transverse in nature.

Question 89

Which of the following amino acids exhibits the largest number of resonance structures?

A. Lysine

B. Isoleucine

C. Glutamate

D. Valine

Answer 89

C. Glutamate

Since all standard amino acids share the same backbone, we simply need to compare the side chains of these four choices. Glutamate, the conjugate base of glutamic acid, has a –COO- functionality as part of its side chain. As such, it possesses an added resonance structure in comparison to the other three species.

Question 90

With regard to leaving group alone, which of these species is the most unstable?

A. Acetyl fluoride

B. Acetyl chloride

C. Acetyl bromide

D. Acetyl iodide

Answer 90

D. Acetyl iodide

Of the four halogens included in these molecules, iodine serves as the best leaving group. To understand this concept, consider the periodic table. As a member of a much lower period than (for example) fluorine, iodine is a very large atom. As such, it can easily delocalize the negative charge gained when it exits as a leaving group. The better the leaving group, the more reactive the compound, and a more reactive molecule is by definition less stable.

Acetyl fluoride represents the least reactive, and thus most stable, acyl halide listed.

Question 91

Which of the following amino acid sequences would incur the greatest entropic penalty if it were used to replace Tyr-Cys-Met in the surface region of a protein?

A. His-Gly-Gly

B. Ala-Gly-Ser

C. Leu-Val-Phe

D. Met-Thr-Glu

Answer 91

C. Leu-Val-Phe

Entropic penalties are incurred when amino acids are exposed to an environment in which they are poorly soluble. In this case, the surface of a protein is likely to face an aqueous environment, so hydrophilic residues are favored and hydrophobic residues incur a penalty. Leucine, valine and phenylalanine are all hydrophobic amino acids.

Question 92

Under standard conditions, which of the following is FALSE regarding 1 L of pure water?

A. K_w = 10^-14

B. pH = 7

C. pH + pOH = 7

D. [H⁺] = [OH^-]

Answer 92

C. pH + pOH = 7

Remember, pH + pOH = 14 under standard conditions.