Edexcel Jun 2013 Paper 4 Q18 (with explained solutions) |

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Edexcel Jun 2013 Paper 4 Q18

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18 Ethanedioic acid, H2C2O4, is a dicarboxylic acid which occurs in many plants, for example in rhubarb leaves, and is used as a rust remover and strong descaler. The structure of ethanedioic acid is shown below. O O H O C C O H Ethanedioic acid is a much stronger acid than carboxylic acids such as ethanoic acid, having a pKa of 1.38. The hydrogenethanedioate ion, HC2O4 , is a weaker acid than ethanedioic acid, having a pKa of 4.28, although slightly stronger than ethanoic acid. (a) (i) Write an equation for the reaction of the hydrogenethanedioate ion with water to form an acidic solution. Include state symbols in your equation. (ii) Write the expression for the acid dissociation constant, Ka, of the weak acid, HC2O4 . (2) (1) 12 *P41572A01224*<br />
(iii) A solution containing hydrogenethanedioate ions behaves as a typical weak acid. Use your answer to (a)(ii) and the pKa of the hydrogenethanedioate ion to calculate the pH of a 0.050 mol dm3 solution of sodium hydrogenethanedioate, NaHC2O4. (b) (i) State two approximations used in the calculation of pH in (a)(iii). (3) (2) 1.2.*(ii) Explain why the calculation of the pH of a solution of sodium hydrogenethanedioate gives a more accurate value than a similar calculation for ethanedioic acid. (2)*P41572A01324* 13 Turn over<br />
(c) 25 cm3 of a 0.050 mol dm3 solution of sodium hydrogenethanedioate was titrated with a sodium hydroxide solution of the same concentration. (i) On the axis below, sketch the curve for this titration. pH 14 12 10 20 40 Volume of NaOH / cm3 60 *(ii) When 25 cm3 of a 0.050 mol dm3 solution of ethanedioic acid is titrated with sodium hydroxide solution of the same concentration using phenolphthalein as the indicator, the end point is 50 cm3. When methyl yellow indicator is used, the colour changes at around 25 cm3. Using the information given at the start of the question and quoting data from page 19 of your data booklet, suggest why these volumes are different. (3) (2)(Total for Question 18 = 15 marks) 14 *P41572A01424*<br />

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Question Number Acceptable Answers Reject Mark Reject Mark Ka = [H+]2 / [HC2O4 [H+][A]/ [HA] ] 2(aq) + H3O+(aq) (aq) + H2O(l)C2O4 18(a)(i) HC2O4 (or ) ALLOW H2O(aq) Equation (1) states (1) ALLOW for 1 mark (aq)C2O4 HC2O4 States mark is not stand alone but can be awarded if the equation has a minor error e.g. an incorrect charge 2(aq) + H+(aq) Acceptable Answers Question Number 18(a)(ii) Ka = [C2O4 2] [H3O+] / [HC2O4 ] OR Ka = [C2O4 No TE on incorrect equation in (a)(i) Penalise incorrect charges in (i) and (ii) once only 2] [H+] / [HC2O4 ] Question Number 18 (a) (iii) Reject Mark Acceptable Answers ] No TE on (a)(ii) Ka = 104.28 OR 5.24807 x 10-5 (mol dm3) (1) Ka = [H+]2 / [HC2O4 Ka = [H+]2 / 0.050 [H+] = (0.05 x 104.28) = 1.61988 x103 (mol dm3) (1) TE on incorrect Ka value pH = log 1.61988 x 103 = 2.7905 = 2.8 (1) For final mark TE on algebraic / arithmetical errors providing pH1.3 Correct answer with no working scores 3 Ignore SF except 1 SF 6CH04011306<br />
Question Number 18(b)(i) Acceptable Answers Reject Mark Use of NaHC2O4 for HC2O4 OR sodium hydrogen- ethanedioate for hydrogen- ethanedioate ion throughout this item ]equilibrium = [HC2O4 /acid /hydrogenethanedioate ion ionization IGNORE explanations First mark: HC2O4 negligible ALLOW Acid for HC2O4 Slight / partial / incomplete / does not dissociate for negligible OR ]initial / 0.050 (mol dm3) [HC2O4 (1) Second mark: [H+] due to ionization of water negligible OR auto ionization of water negligible OR [H+] only due to ionization of HC2O4 OR [C2O4 IGNORE references to temperature and to HA and A Penalize omission of [] in discussion once only 2] = [H+] (1) 6CH04011306<br />
Question Number 18(b)(ii) Ethanedioic acid is a (much) stronger acid (than Acceptable Answers hydrogenethanedioate ion / sodium hydrogenethanedioate) OR Ethanedioic acid has a (much) smaller pKa (than hydrogenethanedioate) OR Ionization / dissociation of ethanedioic acid is (much) greater (than hydrogenethanedioate) OR Reverse arguments (1) IGNORE NaHC2O4 ionization negligible Approximation of negligible ionization invalid / incorrect OR [H2C2O4]equilibrium not equal to [H2C2O4]initial (1) No TE on 18(a)(iii) IGNORE Second ionization occurs Question Number 18(c)(i) Start pH at 2.8 Acceptable Answers ALLOW 24 (1) Vertical section at 25 cm3 within pH range 6-11 and 2.5-4 units long (1) end pH (approaching) value in range 12-13 (asymptotically) (1) Mark Reject Ethanedioic acid is a strong acid / fully dissociated Just approximation invalid Reject Mark deviation from vertical maximum before final pH 6CH04011306<br />
Question Number 18(c)(ii) First mark: Acceptable Answers Reject Mark Methyl yellow range = 2.94 and the phenolphthalein range = 8.210 ALLOW pKin (methyl yellow) = 3.5 and pKin (phenolphthalein) = 9.3 (1) Second mark: (The volumes are different) because ethanedioic acid is dibasic / diprotic / has two replaceable/acidic hydrogen atoms ALLOW dicarboxylic (acid) (therefore there are two stages to the neutralization) OR Methyl yellow range coincides with neutralization of first proton and phenolphthalein range coincides with neutralization of second proton (1) Total for Question 18 = 15 Marks 6CH04011306<br />

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