At the half-equivalence point in the titration of a weak monoprotic acid (HA) with a strong base (NaOH), what relationship exists between pH and pKa?
Answer and explanation
Correct answer: C. pH = pKa
At the half-equivalence point, exactly half of the initial weak acid HA has been neutralized by added hydroxide into its conjugate base A-. Therefore, [HA] = [A-]. In the Henderson-Hasselbalch equation pH = pKa + log([A-]/[HA]), the log term becomes log(1) = 0, so pH = pKa.
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