MCAT Chemistry 4 — Questions and Answers
Question 1: Which of the following describes Le Chatelier's principle?
- A system at equilibrium will shift to counteract any imposed change in conditions (Correct answer)
- The rate of the forward reaction always equals the rate of the reverse reaction
- The equilibrium constant changes when temperature is altered
- Catalysts shift the equilibrium position toward products
Correct answer: A system at equilibrium will shift to counteract any imposed change in conditions
Le Chatelier's principle states that a system in equilibrium will respond to stress (changes in concentration, pressure, or temperature) by shifting to partially counteract the disturbance.
Question 2: In organic chemistry, which functional group is present in amines?
- C=O (carbonyl)
- –OH (hydroxyl)
- –NH₂ (amino) (Correct answer)
- –COOH (carboxyl)
Correct answer: –NH₂ (amino)
Amines contain a nitrogen atom bonded to hydrogen and/or carbon atoms; primary amines have the –NH₂ functional group.
Question 3: Which of the following best describes what happens during saponification?
- An ester reacts with a strong base to form a carboxylate salt and an alcohol (Correct answer)
- A carboxylic acid reacts with an alcohol to form an ester and water
- An alcohol is oxidized to form a carboxylic acid
- A ketone is reduced to form a secondary alcohol
Correct answer: An ester reacts with a strong base to form a carboxylate salt and an alcohol
Saponification is the base-catalyzed hydrolysis of an ester, producing a carboxylate salt and alcohol; it is the chemical basis of soap-making from triglycerides.
Question 4: Which of the following molecules is most likely to exhibit resonance?
- CH₄
- H₂O
- C₆H₆ (benzene) (Correct answer)
- NaCl
Correct answer: C₆H₆ (benzene)
Benzene exhibits resonance because its delocalized π electrons can be represented by multiple equivalent Lewis structures with alternating single and double bonds.
Question 5: A solution contains 0.1 mol/L of a weak acid with Ka = 1×10⁻⁵. What is the approximate pH?
- 1.0
- 3.0 (Correct answer)
- 5.0
- 7.0
Correct answer: 3.0
For a weak acid, [H⁺] ≈ √(Ka × C) = √(1×10⁻⁵ × 0.1) = √(1×10⁻⁶) = 1×10⁻³, so pH = 3.
Question 6: Which of the following is an example of a nucleophile in organic chemistry?
- BF₃
- AlCl₃
- H₂O (Correct answer)
- H⁺
Correct answer: H₂O
Water (H₂O) is a nucleophile because it has lone pairs of electrons on oxygen that it can donate to electrophilic carbon atoms.
Question 7: What is the relationship between cell potential (E°cell) and the Gibbs free energy change (ΔG°)?
- ΔG° = nFE°cell
- ΔG° = -nFE°cell (Correct answer)
- ΔG° = RT ln E°cell
- ΔG° = -RT/nF × E°cell
Correct answer: ΔG° = -nFE°cell
The relationship is ΔG° = -nFE°cell, where n is moles of electrons transferred and F is Faraday's constant; a positive E°cell gives a negative (favorable) ΔG°.
MCAT Chemistry 4
Which of the following describes Le Chatelier's principle?