8.9 Henderson-Hasselbalch Equation

Identify the pH of a buffer solution based on the identity and concentrations of the conjugate acid-base pair used to create the buffer.

• The pH of the buffer is related to the pKa the acid and the concentration ratio of the of conjugate acid-base pair. This relation is a consequence of the equilibrium expression associated with the dissociation of a weak acid, and is described by the HendersonHasselbalch equation. Adding small amounts of acid or base to a buffered solution does not significantly change the ratio of [A−]/[HA] and thus does not significantly change the solution pH. The change in pH on addition of acid or base to a buffered solution is therefore much less than it would have been in the absence of the buffer. EQN: pH = pKa + [] − log [A ] HA
  • Exclusion: Computation of the change in pH resulting from the addition of an acid or a base to a buffer will not be assessed on the AP Exam. Derivation of the Henderson-Hasselbalch equation will not be assessed on the AP Exam.