Are HSO4− and SO4^2− a conjugate acid–base pair?

• A. Yes; HSO4− is the conjugate acid of SO4^2−
• B. No; they are not related
• C. Yes; SO4^2− is the conjugate acid of HSO4−
• D. Yes; they are conjugate bases of each other

Answer: (A)

Conjugate acid–base pairs are two species related by the gain or loss of a single proton. The base SO4^2− can gain a proton to become HSO4−, so HSO4− is the conjugate acid of SO4^2−. In the reverse direction, HSO4− can donate a proton to form SO4^2−, making SO4^2− the conjugate base of HSO4−. Because they are connected by that one-proton transfer, they form a conjugate acid–base pair.