A-LevelChemistryEquilibria9701/42 · Feb/Mar 2019 · Q37 Marks
Use mathematical expressions to define the following terms. • pH = • Ka for a weak acid, HA =
- (a)(i)Use mathematical expressions to define the following terms. • pH = • Ka for a weak acid, HA =[2]
- (a)(ii)Write equations to show how a buffer solution consisting of a mixture of HA(aq) and NaA(aq) controls pH when an acid or an alkali is added.[2]
- (b)When chlorine dissolves in water the following reaction occurs. Cl₂(g) + H₂O(l) → HClO(aq) + H⁺(aq) + Cl⁻(aq) When solutions of chlorine are used for water purification, the pH of the solution of chlorine is kept near to pH 7 by the addition of a base. Chlorine is dissolved in water to produce 1000 cm³ of a solution containing 0.170 mol of HClO and 0.170 mol of HCl. A buffer solution is then prepared by adding 0.200 mol of NaOH(s) to this solution. The NaOH reacts initially with the HCl. Calculate the pH of the buffer solution. [HClO is a weak acid with Ka = 2.9 × 10⁻⁸ mol dm⁻³.] pH =[3]
Mark scheme
Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.
- [2] M1 pH = -log[H⁺]
- M2 Ka = [H⁺][A⁻]/[HA]
- [2] M1 A⁻ + H⁺ → HA OR NaA + H⁺ → HA + Na⁺
- M2 HA + OH⁻ → H₂O + A⁻
- [3] remaining n(HClO) = 0.17 – 0.03 = 0.14 mol (dm⁻³)
- [H⁺] = 1.35 x 10⁻⁷ mol dm⁻³ OR calculate pKa (7.54) from Ka
- pH = -log(1.35 × 10⁻⁷) = 6.87/6.9_OR_pH = 7.54 + log(0.03/0.14) = 6.87
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