For a monoprotic weak acid titration with a strong base, the pH of the solution undergoes characteristic changes as titrant is added.
(a) Describe, in chemical terms, what is present in significant amounts at the half-equivalence point of such a titration.
(b) Explain why the pH at the half-equivalence point depends only on the intrinsic properties of the acid and not on its initial concentration.
(c) Compare the pH at the half-equivalence point with the pH at the stoichiometric equivalence point for a weak acid titration. Explain the difference in terms of the predominant species present and any relevant chemical equilibria.
(d) A student claims that a weak acid with a smaller pKa will reach its equivalence point at a lower pH. Evaluate the validity of this claim and justify your answer.
(a) Identify the major species at the half-equivalence point.
(b) Explain why the half-equivalence pH is independent of initial acid concentration.
(c) Contrast the pH at half-equivalence with the pH at equivalence for a weak acid titration.
(d) Evaluate and justify the student’s claim regarding pKa and equivalence-point pH.