The Henderson–Hasselbalch equation is applicable in buffer solutions where both conjugate base and weak acid are present. The equation is:

• A. pH = pKa + log([HA]/[A−])
• B. pH = pKa + log([A−]/[HA])
• C. pH = pKa − log([A−]/[HA])
• D. pH = pKa × log([A−]/[HA])

Answer: (B)

Buffers rely on a weak acid HA and its conjugate base A−. The pH of such a system is governed by how strong the acid is (its pKa) and the relative amounts of the base form to the acid form. The Henderson–Hasselbalch relationship tells us pH = pKa + log([A−]/[HA]), using the base-10 logarithm.

This means that when the conjugate base is present in larger amounts, the ratio [A−]/[HA] is greater than 1, the log term is positive, and the pH rises above pKa. If the acid form dominates, the ratio is less than 1, the log term is negative, and the pH drops below pKa. When the two forms are present in equal amounts, pH equals pKa.

Using the reciprocal ratio or multiplying by log terms would give the wrong directional relationship between the ratio and pH, so those forms don’t correctly describe buffer behavior.