Edexcel Jun 2015 (IAL) Paper 6 Q3 (with explained solutions) |

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Edexcel Jun 2015 (IAL) Paper 6 Q3

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3 Cupronickel is an alloy of copper and nickel. It is used to make silver coins. A coin is analysed by the following method. Step 1 Step 2 It is weighed on a balance which reads to two decimal places and found to have mass 4.00 g. Water is added to the coin in a beaker. Concentrated nitric and sulfuric acids are added and the coin dissolves. Step 3 When the coin is completely dissolved, the solution is neutralized. Step 4 Step 5 The neutral solution is transferred, with the washings, to a 100 cm3 volumetric flask, made up to the mark with water and mixed thoroughly. 10 cm3 samples of the solution are taken and an excess of potassium iodide is added, producing iodine. Step 6 The iodine is titrated with 0.200 mol dm3 sodium thiosulfate solution. (a) Why, in Step 2, is water added before, rather than after, the acids? (1)(b) What is the colour of an aqueous iodine solution? (1)(c) (i) To make the end point of the titration more obvious, an indicator is added just before the colour of the iodine disappears. Name this indicator. (1)(ii) Suggest why the indicator is not added to the iodine solution earlier in the titration. (1)(iii) Give the colour change at the end point when the indicator is used in this titration. (1)*P44884A0616*<br />
(d) The results for the titrations are shown below. 24.10 0.00 47.90 24.10 23.55 0.00 47.00 23.55 Titration number Burette reading (final) / cm3 Burette reading (initial) / cm3 Titre / cm3 (i) Complete the table. (ii) Which titres should be used to calculate the mean? Explain your choice. (1) (1)(iii) Calculate the mean titre. (1) *P44884A0716* Turn over<br />
(iv) Calculate the percentage by mass of copper in the coin. Use the equations below. 2Cu2+(aq) + 4I(aq) (cid:111) 2CuI(s) 2S2O3 2(aq) + I2(aq) (cid:111) S4O6 + I2(aq) 2(aq) + 2I(aq) (5) *P44884A0816*<br />
(v) The uncertainty in each burette reading is 0.05 cm3 and the uncertainty in each reading of the balance is 0.005 g. Calculate the percentage uncertainty in the third titre value and in the mass measurement. Use your results to decide whether using a balance that weighs to three decimal places would significantly improve the accuracy of the result. (2) (Total for Question 3 = 15 marks) *P44884A0916* Turn over<br />

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Question Number 3(a) Question Number 3(b) Question Number 3(c)(i) Question Number 3(c)(ii) Mark Acceptable Answers Reject Sulfuric acid reacts very exothermically with water ALLOW The reaction with acid is exothermic OR (Sulfuric) acid will shoot out of container OR The reaction of water with (sulphuric) acid is vigorous/splashes OR Prevent splashing of acid nitric acid Prevent splash alone Acceptable Answers Reject Mark (Pale/straw-coloured) yellow / brown / red-brown Red/purple/blue/black/blue- black/orange Acceptable Answers Reject (Freshly prepared) starch (solution) Mark Acceptable Answers Reject Mark (If starch is added too early) starch iodine complex formed (doesnt re-dissolve) ALLOW Iodide for iodine ALLOW (Black) solid/precipitate / complex forms OR Insoluble compound forms WCH0601 1506<br />
Question Number 3(c)(iii) Question Number 3(d)(i) Question Number 3(d)(ii) Acceptable Answers Reject Mark to clear From blue-black to colourless ALLOW From blue / black/ dark blue/ deep blue to colourless Mark Mark Acceptable Answers Reject 24.1(0) 23.8(0) 23.55 23.45 (cm3) Reject Acceptable Answers The third and fourth / 23.55 and 23.45 (cm3) and They are concordant OR Within 0.2/0.1 (cm3) IGNORE Anomalous WCH0601 1506<br />
Question Number 3(d)(iii) 23.5(0) (cm3) Acceptable Answers Reject Mark ALLOW TE including second titre value, mean = 23.6(0) (cm3) Question Number 3(d)(iv) Reject Mark Acceptable Answers Correct answer 74.6% / 75% OR 74.9% (TE from 23.60 average titre) Ignore SF except 1SF With no working (5) Number of mol of thiosulfate = 23.50 x 0.200 (1) 1000 = 4.70 x 10-3/0.00470 Second mark EITHER Number of mol of iodine = 4.70 x 10-3 2 = 2.35 x 10-3/0.00235 AND Number of moles of copper ion = 2 x 2.35 x 10-3 (1) = 4.70 x 10-3/0.00470 in 10 cm3 OR From equations amount of iodine is half amount of thiosulfate and amount of copper is twice amount of iodine, so amount of copper equals amount of thiosulfate for this mark Number of moles of copper in solid WCH0601 1506<br />
= 10 x 4.70 x 10-3 (1) = 4.70 x 10-2/0.0470 Mass of copper in solid = 4.70 x 10-2 x 63.5 (g) (1) = 2.9845 Percentage copper = 2.9845 x 100 4.00 = 74.6125 = 74.6% (1) Using 23.60 by averaging titres 2, 3 and 4 4.72 x 10-3/0.00472 (1) 2.36 x 10-30.00236 AND 4.72 x 10-3/0.00470 (1) 4.72 x 10-2/0.0470 (1) 2.9972 (1) 74.9% (1) Answers greater than 100% max 3 WCH0601 1506<br />
Question Number 3(d)(v) Mark Acceptable Answers Reject First Mark Uncertainty in titre value: 2 x 0.05 x 100 = 23.55 ()0.42/0.425/0.4246% (1) Second Mark Uncertainty in the mass measurement: 2 x 0.005 x 100 = ()0.25% 4.0 OR 1 x 0.005 x 100 = ()0.125% 4.0 so it would / would not be worth using a 3 dp balance (1) Ignore SF including 1 SF (Total for Question 3 = 15 marks) WCH0601 1506<br />

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