14 (1410U30-1) 10. A student used a solution of weak acid HA in a titration. A 25.0 cm3 sample of the acid was titrated against a sodium hydroxide solution of concentration 0.250 mol dm–3 giving the pH titration curve shown. The initial pH value is missing from the graph. © WJEC CBAC Ltd. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 5 10 15 20 25 30 35 40 45 Volume of sodium hydroxide added / cm3 pH 14 (1410U30-1) Turn over. 15 Examiner only (a) Several indicators are listed in the table. Identify all the indicator(s) which are appropriate for this titration. Give a reason for your choice(s). [2] © WJEC CBAC Ltd. Indicator pH range methyl orange 3.1 - 4.4 bromocresol purple 5.2 - 6.8 bromothymol blue 6.0 - 7.6 naphtholphthalein 7.3 - 8.7 cresolphthalein 8.2 - 10.1 (b) Calculate the concentration of the HA solution. [2] Concentration = ....................................................... mol dm–3 15 16 (1410U30-1) Examiner only (c) Find the pH of the mixture after addition of 10.0 cm3 of sodium hydroxide solution and hence calculate the initial pH of the HA solution. [4] You must show your working. pH after addition of 10.0 cm3 = ....................................................... Initial pH = ....................................................... (d) HA is a weak acid and so it can be used to make a buffer solution. Suggest one important use for buffer solutions. [1] © WJEC CBAC Ltd. 16 (1410U30-1) Turn over. 17 Examiner only (e) Some data regarding the dissociation of another weak acid, HX, at two different temperatures are given in the table. © WJEC CBAC Ltd. HX H+ X– + HX at 298 K HX at temperature T Percentage dissociated 3.9 3.4 ΔG for acid dissociation / kJ mol–1 15.9 16.7 ΔH for acid dissociation / kJ mol–1 –10.0 ΔS for acid dissociation / J K–1 mol–1 –87.0 Find temperature T and hence explain in terms of equilibrium why the percentage dissociation is different at this temperature. [3] T = ....................................................... K 12 17