Edexcel Jan 2013 Paper 5 Q15 (with explained solutions) |

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Edexcel Jan 2013 Paper 5 Q15

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15 Vitamin C (L-ascorbic acid) is present in fresh fruit and vegetables although prolonged cooking destroys it. The structure of ascorbic acid, C6H8O6, is shown below. HO HO OH HO (a) The amount of ascorbic acid present in a sample is determined by reacting it with a known amount of iodine. The excess iodine is then measured by titration with a solution of sodium thiosulfate, using a starch indicator. The equations for the reactions are given below. HO HO + I2 HO OH HO HO + 2H+ + 2I 2S2O3 2 + I2 S4O6 2 + 2I Four 500 mg vitamin C tablets were dissolved in distilled water and the solution made up to 250 cm3 in a volumetric flask. A 25.0 cm3 portion of this solution was added to an iodine solution containing 2.00103 moles of iodine. The resulting mixture was titrated with sodium thiosulfate solution of concentration 0.0631 mol dm3. The titration was repeated and the mean (average) titre was 27.85 cm3. (i) State the type of chemical change undergone by ascorbic acid when it reacts with iodine. (1)14 *P41215A01424*<br />
(ii) State the colour of the solution in the flask just before the starch is added to the titration mixture and state the colour change at the end-point of the titration. (2)(iii) Calculate the percentage by mass of ascorbic acid in the tablets. The molar mass of ascorbic acid is 176 g mol1. (5) *P41215A01524* 15 Turn over<br />
(iv) Explain why using four tablets in 250 cm3 of solution gives a more accurate result than two tablets in 250 cm3. (1)(b) The structure of ascorbic acid is shown again below. Vitamin C is one of the optical isomers of this structure. HO HO OH HO (i) Mark on this diagram the two chiral centres of this molecule. (ii) How might you show that vitamin C is a single optical isomer and not a racemic mixture of the optical isomers of ascorbic acid? (2) (2)(iii) Despite its name, ascorbic acid is not a carboxylic acid but it does contain an ester group. Suggest what happens to destroy vitamin C on prolonged cooking. (1)(Total for Question 15 = 14 marks) 16 *P41215A01624*<br />

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Mark Mark Mark Acceptable Answers Reject (vitamin C / ascorbic acid ) oxidation / oxidized / oxidised ALLOW oxidisation Acceptable Answers (very) pale yellow / straw coloured (1) IGNORE just before the end-point blue-black to colourless (both needed) (1) Accept (dark) blue or black ALLOW pale yellow / straw coloured to colourless for 1/2 Redox / oxidation- reduction / reduction-oxidation Reject Just yellow Clear Acceptable Answers Reject Answers greater than 100% 22 2 = 27.85 x 103 x 0.0631 (1) Moles S2O3 (= 1.757335 x 103) moles of I2 remaining = Moles S2O3 = 27.85 x 103 x 0.06312 = 8.786675 x 104 = 8.79 x 104 (1) Moles ascorbic acid = moles I2 at startmoles I2 remaining = 2.00 x 1038.786675 x 104 = 1.1213325 x 103 = 1.12 x 103 (1) Mr (ascorbic acid) = 176 Mass ascorbic acid in 250 cm3 = 10 x Mr x moles ascorbic acid = 10 x 176 x 1.1213325 x 103 (1) (= 1.97355) Percentage ascorbic acid in tablet 100 x mass ascorbic acid in 250 cm32 = 100 x 10 x 176 x 1.1213325 x 1032 = 98.67726 = 98.7% (1) IGNORE SF except 1 SF Premature rounding gives 98.5% (5) Correct answer with no working scores full marks TE at each stage of the calculation. Question Number 15 (a)(i) Question Number 15 (a)(ii) Question Number 15 (a)(iii)<br />
Mark Reject Just reduces the percentage error Titration value will be larger (with four tablets) so reduces the percentage error (in volume measurement) Reject Mark Reject Mark Question Number 15(a)(iv) EITHER Acceptable Answers Using larger mass reduces the percentage error / uncertainty (in weighing) OR Using larger amount reduces the percentage error / uncertainty in weighing OR Reverse discussion of two tablets ALLOW using four tablets gives a more representative sample Acceptable Answers HO HO HO OH (2) Mark independently ALLOW any clear indication of chiral centres Acceptable Answers First mark Use of (plane-)polarized light (mentioned somewhere) (1) ALLOW Use a polarimeter Second mark Pure optical isomer / enantiomer) rotates the plane of (plane-) polarized light OR racemic mixture has no effect on the plane of (plane-) polarized light (1) IGNORE optically active / inactive ALLOW rotates plane-polarized light scores 2 Question Number 15 (b)(i) Question Number 15 (b)(ii)<br />
Reject Mark C=O is broken Just oxidation Total for Q15 =14 Marks Question Number 15(b)(iii) (Ester group / vitamin C / it) is hydrolysed Acceptable Answers ALLOW Vitamin C is oxidized Ester / vitamin C is broken down to form carboxylic acid and alcohol (groups) IGNORE Just breaks down<br />

Question 14 15 16 17

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