Which equation is used to calculate the pH of a buffer solution?

The correct equation for calculating the pH of a buffer solution is based on the Henderson-Hasselbalch equation, which in its most widely used form is:

pH = pKa + log([A⁻]/[HA])

In this context, [A⁻] refers to the concentration of the conjugate base of the weak acid (HA) in the buffer solution, while [HA] represents the concentration of the weak acid itself. This equation is crucial for understanding buffer systems because it highlights how the ratio of the concentrations of the acid and its conjugate base can influence the pH of the solution.

A buffer solution resists changes in pH upon the addition of small amounts of acid or base. The Henderson-Hasselbalch equation demonstrates the relationship between the pH of the buffer, the pKa (which is the negative logarithm of the acid dissociation constant, Ka), and the ratio of the concentrations of the weak acid and its conjugate base. Thus, the effective buffer capacity is maintained as long as both [HA] and [A⁻] are present in sufficient amounts.

The other choices, although relevant to pH calculations or buffer systems, do not accurately represent the relationship needed for a