The steep rise in pH near equivalence is due to:

• A. The acid still present keeps pH low.
• B. A small amount of added base converts large fractions of HA to A-, causing rapid pH increase.
• C. The base is strong enough to neutralize everything immediately.
• D. The buffer pair absorbs all base with no pH change.

Answer: (B)

The essential idea is how the balance between the weak acid (HA) and its conjugate base (A−) controls pH as base is added. In a HA/A− buffer, pH is governed by the ratio [A−]/[HA] (via pH ≈ pKa + log([A−]/[HA])). As you add a small amount of base, HA is converted to A−, so the ratio [A−]/[HA] grows. Since [HA] is being depleted, even a tiny bit of added base causes a relatively large change in this ratio, leading to a rapid increase in pH. Near the equivalence point, almost all the HA has been converted to A−, so a little more base pushes the pH up quickly. After equivalence, the solution is dominated by A−, which hydrolyzes to produce OH−, keeping the pH high.

That’s why the pH rises steeply near equivalence—the small amount of added base shifts the HA ⇌ A− balance dramatically, and the pH responds strongly to that change.