Question 116 marks
Question
(a) Disodium phosphate, (Na+)2(HPO42–), reacts with an acid to form monosodium phosphate, Na+(H2PO4–). (i) Identify the ions that are a conjugate acid–base pair in this reaction, using the formulae of the species involved. conjugate acid conjugate base [1] (ii) Define buffer solution. …… [2] (iii) Write two equations to show how a mixture of (Na+)2(HPO42–) and Na+(H2PO4–) can act as a buffer solution. equation 1 …… equation 2 …… [2] (iv) Identify one inorganic ion that acts as a buffer in blood. …… [1] (b) Compound E is the hydroxide of a Group 2 element. Compound E is a strong alkali. 2.63 g of E is dissolved in water to make 250 cm3 of solution F. Solution F has a pH of 13.09 at 298 K. (i) Show that the concentration of hydroxide ions in solution F is 0.123 mol dm–3. [2] (ii) Explain why the concentration of compound E in solution F is 0.0615 mol dm–3. …… [1] (iii) Use the concentration given in (ii) to identify compound E. compound E …… [1] (c) Compound E is much more soluble than magnesium hydroxide. A saturated solution of magnesium hydroxide in water has a concentration of 1.40 × 10–4 mol dm–3 at 298 K. Calculate the solubility product, Ksp, of magnesium hydroxide. Include units. Ksp = …… units …… [3] (d) Explain why compound E is much more soluble than magnesium hydroxide. …… [3] [Total: 16]
Mark scheme
1(a)(i) conjugate acid conjugate base 1 H2PO4– HPO42– [1] BOTH 1(a)(ii) allows only small changes in pH / resists changes in pH [1] 2 when small amounts of acid or alkali/base / H+ or OH– are added [1] 1(a)(iii) Na+2HPO42– + H+ → Na+H2PO4– + Na+ 2 or HPO42– + H+ → H2PO4– [1] Na+H2PO4– + NaOH → Na+2HPO42– + H2O or H2PO4– + OH– → HPO42– + H2O [1] 1(a)(iv) HCO3– / hydrogen carbonate [1] 1 1(b)(i) either 10–13.09 or [H+]= 8.1 × 10–14 seen [1] 2 8.1x10–14 × [OH–] = 1 × 10–14 seen [1] 1× 10−14 OR [OH–] = seen 8.1× 10 −14 ALLOW alternative method pOH = 0.91 [1] [OH–] = 10–0.91 seen [1] 1(b)(ii) 2 moles of OH– in one mole of E / X(OH)2 1 OR there are two hydroxide ions in each formula unit of E / X(OH)2 [1] 1(b)(iii) 0.0615 × 0.25 = 0.0154 moles of E 1 RFM = 2.63 / 0.0154 = 170.8 / 171 OR RAM = 34 – 2.63 / 0.0154 = 137.1 /137 AND barium hydroxide / Ba(OH)2 [1] 1(c) Ksp = [Mg2+][OH–]2 OR Ksp = 1.4 × 10-4 × (2 × 1.4 × 10-4)2 [1] 3 Ksp = 1.10 × 10–11 [1] min 2sf correct answer [2] mol3 dm–9 [1] ecf from M1 1(d) M1: ΔHlatt and ΔHhyd less exothermic (down the group) 3 OR ΔHlatt/LE and ΔHhyd more exothermic/more negative for Mg [1] M2: ΔHlatt changes more than ΔHhyd (down the group) OR ΔHlatt changes more / faster for Mg [1] M3: ΔHsol becomes more exothermic (down the group) OR ΔHsol becomes less exothermic / less negative for Mg [1] mark independently