The half-equivalence point occurs when:

The half-equivalence point occurs when:

• A. The added base volume equals half the volume needed to reach equivalence.
• B. The pH equals 7.
• C. All HA is converted to A-.
• D. The solution contains only A-.

Answer: (A)

In a weak acid–strong base titration, the half-equivalence point is when exactly half of the original acid has been neutralized. At this stage, the concentrations of the weak acid (HA) and its conjugate base (A−) are equal, so the solution forms a buffer with equal amounts of HA and A−. For a buffer, the Henderson–Hasselbalch relationship tells us pH = pKa when [HA] = [A−], which is why the pH at the half-equivalence point equals the acid’s pKa. This is also why the amount of base added at half-equivalence is half of what’s needed to reach full neutralization (the equivalence point). The other statements don’t fit this situation: pH = 7 is not inherently true for half-equivalence unless the acid’s pKa is 7, and it’s not a universal rule. Saying all HA is converted would describe the equivalence point, not half-equivalence. Saying the solution contains only A− would occur only after complete neutralization, well past half-equivalence.

In a weak acid–strong base titration, the half-equivalence point is when exactly half of the original acid has been neutralized. At this stage, the concentrations of the weak acid (HA) and its conjugate base (A−) are equal, so the solution forms a buffer with equal amounts of HA and A−. For a buffer, the Henderson–Hasselbalch relationship tells us pH = pKa when [HA] = [A−], which is why the pH at the half-equivalence point equals the acid’s pKa. This is also why the amount of base added at half-equivalence is half of what’s needed to reach full neutralization (the equivalence point).

The other statements don’t fit this situation: pH = 7 is not inherently true for half-equivalence unless the acid’s pKa is 7, and it’s not a universal rule. Saying all HA is converted would describe the equivalence point, not half-equivalence. Saying the solution contains only A− would occur only after complete neutralization, well past half-equivalence.