MCAT General Chemistry Question 235: Answer and Explanation

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Question: 235

4. List the following compounds by increasing pK_a:

I. H₂SO₄

II. NH₃

III. CH₃CH₂COOH

IV. HF

A. I < III < II < IV
B. I < IV < III < II
C. III < I < IV < II
D. II < III < IV < I

Correct Answer: B

Explanation:

B A higher pK_a means a weaker acid, while a lower pK_a means a stronger acid. Since this is a ranking question, start with the extremes. Compound I is a strong acid and will have the lowest pK_a, eliminating choices C and D. Compound II is the only base so it will have the largest pK_a and choice A can be eliminated.

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