Which statement explains the steep rise in pH near the equivalence point?

Which statement explains the steep rise in pH near the equivalence point?

• A. Buffer capacity is at maximum, so pH changes minimally.
• B. Once HA is largely consumed, the solution is dominated by A-, and small additions of base cause large pH increases.
• C. The pH equals pKa at the steep rise.
• D. The salt hydrolysis neutralizes added base.

Answer: (B)

The steep rise in pH around the equivalence point comes from the loss of buffering as the weak acid is nearly exhausted and its conjugate base dominates. While there is still some HA present, the base added is no longer being buffered effectively by HA, so a small amount of added base converts more HA to A− and raises the hydroxide concentration. This causes a rapid change in pH because the solution shifts from a mixture that resists pH change to one where the conjugate base A− is present in much higher relative amount than HA. The Henderson–Hasselbalch relationship captures this: as [A−] / [HA] grows large, pH climbs quickly, giving the characteristic steep rise.

The steep rise in pH around the equivalence point comes from the loss of buffering as the weak acid is nearly exhausted and its conjugate base dominates. While there is still some HA present, the base added is no longer being buffered effectively by HA, so a small amount of added base converts more HA to A− and raises the hydroxide concentration. This causes a rapid change in pH because the solution shifts from a mixture that resists pH change to one where the conjugate base A− is present in much higher relative amount than HA. The Henderson–Hasselbalch relationship captures this: as [A−] / [HA] grows large, pH climbs quickly, giving the characteristic steep rise.