Using the Henderson–Hasselbalch equation, when pH equals pKa, what is the ratio of [A-] to [HA]?

• A. 2:1
• B. 1:1
• C. 1:2
• D. 0:1

Answer: (B)

At the heart is the Henderson–Hasselbalch relation: pH = pKa + log([A-]/[HA]). If pH equals pKa, then the difference is zero, so log([A-]/[HA]) = 0. The logarithm is zero only when the ratio [A-]/[HA] is 1, meaning the conjugate base and the acid are present in equal amounts. So the ratio of [A-] to [HA] is 1:1. This is the condition where the buffer contains equal amounts of acid and conjugate base. The other ratios would shift the pH away from pKa: 2:1 would raise pH above pKa by about 0.30, 1:2 would lower it by about 0.30, and 0:1 would imply no conjugate base at all.