In which of the following titrations is the equivalence point at pH 7?

Question

In which of the following titrations is the equivalence point at pH > 7?

BlackTom AI · Accepted Answer

Equivalence point pH depends on the identity of the species present at equivalence and whether they hydrolyze water. For a weak acid HA titrated with a strong base, the equivalence point contains its conjugate base A–, which hydrolyzes to produce OH– and thus gives pH > 7. Relevant knowledge points: - At the equivalence point the acid has been completely neutralized; only the conjugate species (and spectator ions) remain. - A– + H2O ⇌ HA + OH–; the basicity of A– is given by Kb = Kw / Ka. - If Kb > 0 (i.e., Ka is finite for a weak acid), A– hydrolysis generates OH– and the solution is basic (pH > 7). - Strong acid + strong base titrations give pH ≈ 7 at equivalence (at 25 °C) because neither conjugate hydrolyzes. - Weak base titrated with strong acid leaves conjugate acid BH+, which hydrolyzes to produce H3O+ (pH < 7). Step-by-step reasoning with a numerical example: 1. Assume a weak acid with Ka = 1.0×10^–5 is titrated; at equivalence the concentration of A– in solution is c = 0.050 M (example value after dilution). 2. Kb = Kw / Ka = 1.0×10^–14 / 1.0×10^–5 = 1.0×10^–9. 3. For A– hydrolysis: Kb = [OH–]^2 / (c − [OH–]) ≈ [OH–]^2 / c (if [OH–] << c). 4. [OH–] ≈ sqrt(Kb × c) = sqrt(1.0×10^–9 × 0.050) = sqrt(5.0×10^–11) ≈ 7.07×10^–6 M. 5. pOH = −log[OH–] ≈ 5.15, so pH = 14.00 − 5.15 = 8.85 (>7). Why the correct answer is correct: Because the conjugate base formed at the equivalence point of a weak acid–strong base titration hydrolyzes to produce OH–, making the solution basic (pH > 7). Why the other options are incorrect: - b (Weak base + strong acid): leaves conjugate acid BH+ which hydrolyzes to produce H3O+ so pH < 7. - c (Strong base + strong acid): neither conjugate hydrolyzes; pH ≈ 7 at equivalence (at 25 °C). - d is therefore incorrect. Conclusion: The equivalence point is at pH > 7 for (a) Weak acid and strong base, which matches the provided answer.

In which of the following titrations is the equivalence point at pH > 7?单项选择题

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