MCAT General Chemistry Question 147: Answer and Explanation

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Question: 147

12. A solution is prepared with an unknown concentration of a theoretical compound with a K_a of exactly 1.0. What is the pH of this solution?

A. Higher than 7
B. Exactly 7
C. Less than 7
D. There is not enough information to answer the question.

Correct Answer: C

Explanation:

A higher K_a implies a stronger acid. Weak acids usually have a K_a that is several orders of magnitude below 1. The pK_a of a compound is the pH at which there are equal concentrations of acid and conjugate base; the pK_a of this compound would be -log 1 = 0. With such a low pK_a, this compound must be an acid. Therefore, the pH of any concentration of this compound must be below 7.

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