MCAT General Chemistry Question 81: Answer and Explanation

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Question: 81

6. Acetic acid dissociates in solution according to the following equation:

CH₃COOH ? CH₃COO^– + H⁺

If sodium acetate is added to a solution of acetic acid in excess water, which of the following effects would be observed in the solution?

A. Decreased pH
B. Increased pH
C. Decreased pK_eq (pK_a)
D. Increased pK_eq (pK_a)

Correct Answer: B

Explanation:

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

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