Rank the following molecules in order of increasing pK_a (lowest pK_a/most acidic to highest pK_a/least acidic).

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

Answer: (C)

Verified step by step guidance

1. Identify the functional groups present in each molecule. All three molecules are benzoic acid derivatives with different substituents on the benzene ring.
2. Consider the effect of substituents on acidity. Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base, while electron-donating groups (EDGs) decrease acidity.
3. Analyze molecule II, which has a nitro group (NO2) as a substituent. The nitro group is a strong electron-withdrawing group, which will increase the acidity of the carboxylic acid.
4. Examine molecule III, which has a methoxy group (OCH3) as a substituent. The methoxy group is an electron-donating group, which will decrease the acidity of the carboxylic acid.
5. Compare molecule I, which has no additional substituents, to molecules II and III. Molecule I is less acidic than II due to the lack of an EWG, but more acidic than III due to the absence of an EDG. Therefore, the order of increasing pKa is II < I < III.