Which equation describes the pH in the buffer region of a weak acid–strong base titration?

Which equation describes the pH in the buffer region of a weak acid–strong base titration?

• A. pH = pKa + log([A-]/[HA]).
• B. pH = pKa - log([A-]/[HA]).
• C. pH = pH of the strong base.
• D. pH is fixed at 14.

Answer: (A)

In the buffer region of a weak acid–strong base titration, the solution contains the weak acid HA and its conjugate base A−. The pH is governed by the Henderson–Hasselbalch relationship, which connects pH to the acid’s strength (pKa) and the ratio of base to acid: pH = pKa + log([A−]/[HA]). This shows why adding a small amount of base changes the pH gradually: as more A− is formed, the ratio [A−]/[HA] increases, pushing the pH above the pKa. A key point is the half-equivalence condition: when half the acid has been neutralized, [A−] equals [HA], so log([A−]/[HA]) = 0 and pH = pKa. That’s a hallmark of the buffer region. The pH is not simply the pH of a strong base, nor is it fixed at 14; it shifts with the ratio of conjugate base to acid as titration proceeds. The form with the minus sign would misrepresent how the pH changes with increasing base, so it doesn’t describe the buffer correctly.

In the buffer region of a weak acid–strong base titration, the solution contains the weak acid HA and its conjugate base A−. The pH is governed by the Henderson–Hasselbalch relationship, which connects pH to the acid’s strength (pKa) and the ratio of base to acid: pH = pKa + log([A−]/[HA]). This shows why adding a small amount of base changes the pH gradually: as more A− is formed, the ratio [A−]/[HA] increases, pushing the pH above the pKa.

A key point is the half-equivalence condition: when half the acid has been neutralized, [A−] equals [HA], so log([A−]/[HA]) = 0 and pH = pKa. That’s a hallmark of the buffer region. The pH is not simply the pH of a strong base, nor is it fixed at 14; it shifts with the ratio of conjugate base to acid as titration proceeds. The form with the minus sign would misrepresent how the pH changes with increasing base, so it doesn’t describe the buffer correctly.