In a buffer where pH equals pKa, what is the ratio [A−]/[HA]?

In a buffer where pH equals pKa, what is the ratio [A−]/[HA]?

• A. 1
• B. 0
• C. 10
• D. 0.1

Answer: (A)

When pH equals pKa, the conjugate base form and the weak acid form are present in equal amounts. This comes from the Henderson–Hasselbalch equation: pH = pKa + log([A−]/[HA]). If pH = pKa, the log term is zero, so [A−]/[HA] = 10^0 = 1. Therefore the ratio is 1, meaning there are equal concentrations of A− and HA. This balance is what gives buffers around the pKa their distinctive buffering ability, since the two forms can readily neutralize added acid or base.

When pH equals pKa, the conjugate base form and the weak acid form are present in equal amounts. This comes from the Henderson–Hasselbalch equation: pH = pKa + log([A−]/[HA]). If pH = pKa, the log term is zero, so [A−]/[HA] = 10^0 = 1. Therefore the ratio is 1, meaning there are equal concentrations of A− and HA. This balance is what gives buffers around the pKa their distinctive buffering ability, since the two forms can readily neutralize added acid or base.