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Aqueous Equilibria · Part 3 of 4
6 exam-style questions with model answers, plus 8 quick multi-choice questions — every question on this part of the standard, grouped by the 2 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
State the pH at the equivalence point for (a) a strong acid titrated with a strong base and (b) a weak acid titrated with a strong base, and explain the difference in one sentence each.
Explain why a weak acid titration curve has a buffer region, and explain how the pKa of the acid can be read directly from the curve.
A student titrates 25.0 mL of 0.100 mol L−1 HCl and separately 25.0 mL of 0.100 mol L−1 CH3COOH, each with 0.100 mol L−1 NaOH. They observe that both require exactly 25.0 mL of NaOH but that the curves differ markedly. Explain fully why the equivalence volumes are identical while the initial pH, the shape and the equivalence pH all differ. Ka(CH3COOH) = 1.74 × 10−5
State what an acid–base indicator is in chemical terms, and give the rule for selecting a suitable one for a titration.
Explain why phenolphthalein is suitable for titrating ethanoic acid with sodium hydroxide, but methyl orange is not.
A student is told that all three of methyl orange, bromothymol blue and phenolphthalein give acceptable results when titrating hydrochloric acid with sodium hydroxide, but that only one is acceptable for titrating ethanoic acid with sodium hydroxide. Justify both statements, and explain what property of the two titration curves accounts for the difference.