MCAT Chemistry 2 — Questions and Answers
Question 1: Which of the following best describes a buffer solution?
- A solution that resists changes in pH when small amounts of acid or base are added (Correct answer)
- A solution with a pH of exactly 7.0
- A solution containing equal molar concentrations of a strong acid and strong base
- A solution that completely neutralizes any added acid or base
Correct answer: A solution that resists changes in pH when small amounts of acid or base are added
A buffer resists pH changes because it contains a weak acid and its conjugate base (or weak base and conjugate acid) that can absorb added H⁺ or OH⁻.
Question 2: What is the hybridization of the carbon atom in CO₂?
- sp³
- sp²
- sp (Correct answer)
- sp³d
Correct answer: sp
Carbon in CO₂ forms two double bonds with oxygen, requiring two sp hybrid orbitals and two unhybridized p orbitals for the π bonds.
Question 3: Which quantum number describes the shape of an orbital?
- Principal quantum number (n)
- Angular momentum quantum number (l) (Correct answer)
- Magnetic quantum number (mₗ)
- Spin quantum number (mₛ)
Correct answer: Angular momentum quantum number (l)
The angular momentum quantum number (l) determines the shape of the orbital: l=0 is s (spherical), l=1 is p (dumbbell), l=2 is d.
Question 4: A reaction has ΔG° = +20 kJ/mol at 298 K. Which statement is correct?
- The reaction is spontaneous under standard conditions
- The equilibrium constant K < 1 (Correct answer)
- The reaction cannot proceed in the reverse direction
- The reaction is exothermic
Correct answer: The equilibrium constant K < 1
A positive ΔG° means the reaction is non-spontaneous under standard conditions, and since ΔG° = -RT ln K, a positive ΔG° requires K < 1.
Question 5: In an SN2 reaction, the rate depends on which of the following?
- Only the concentration of the substrate
- Only the concentration of the nucleophile
- Both the concentration of the substrate and nucleophile (Correct answer)
- Neither substrate nor nucleophile concentration
Correct answer: Both the concentration of the substrate and nucleophile
SN2 is a bimolecular reaction with a concerted mechanism where the nucleophile attacks as the leaving group departs, so rate = k[substrate][nucleophile].
Question 6: Which of the following compounds would have the highest boiling point?
- CH₄
- CH₃CH₃
- CH₃OH (Correct answer)
- CH₃CH₂CH₃
Correct answer: CH₃OH
Methanol (CH₃OH) has the highest boiling point because hydrogen bonding between OH groups is much stronger than the London dispersion forces in the hydrocarbons.
Question 7: The Henderson-Hasselbalch equation is used to calculate:
- The concentration of a strong acid solution
- The pH of a buffer solution given pKa and the ratio of conjugate base to acid (Correct answer)
- The standard reduction potential of a half-reaction
- The solubility product of a sparingly soluble salt
Correct answer: The pH of a buffer solution given pKa and the ratio of conjugate base to acid
Henderson-Hasselbalch states pH = pKa + log([A⁻]/[HA]), relating buffer pH to the pKa of the weak acid and the ratio of its conjugate base to acid.
MCAT Chemistry 2
Which of the following best describes a buffer solution?