A simplified titration curve is recorded for 0.10 M HA (25.0 mL) titrated with 0.10 M NaOH. The measured pH at 12.5 mL added is 4.8. The concept being tested is identifying from half-equivalence data.
Assuming ideal behavior and a monoprotic acid, which conclusion is most consistent with the data?
- because at half-equivalence . (correct answer)
- because half-equivalence implies neutrality.
- because the conjugate base dominates at half-equivalence.
- cannot be inferred from titration data without calculating .
Explanation: This question assesses understanding of acid-base equilibria in physiological systems (5A). Acid-base equilibria involve the balance between acids and bases in solution, often maintained by buffer systems. In this passage, the system's pH regulation is demonstrated through titration data at half-equivalence. Choice A correctly describes pKa ≈4.8 because pH = pKa at half-equivalence for monoprotic acids. Choice B fails because it assumes neutrality at 7.0, which is a common error when ignoring pKa dependence. In future questions, ensure buffer systems are evaluated by considering both capacity and range, using half-equivalence for pKa estimation.