Which anion is the most basic?

• A. CH3COO-
• B. CN-
• C. NO2-
• D. F-

Answer: (B)

In water, the basicity of an anion is determined by the strength of its conjugate acid—the weaker the acid, the stronger the base. The conjugate acids here are HCN, HF, HNO2, and CH3COOH, with approximate pKa values of 9.2 for HCN, 3.17 for HF, 3.3 for HNO2, and 4.76 for CH3COOH. A high pKa means a weak acid, which makes its conjugate base relatively stronger at grabbing a proton. Since HCN is the weakest acid among these, its conjugate base, CN−, is the strongest base. The others form conjugate acids that are much stronger (lower pKa), so their bases are comparatively weaker. Thus the cyanide ion is the most basic.