Edexcel Jun 2012 Paper 3 Q5 (with explained solutions) |

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Edexcel Jun 2012 Paper 3 Q5

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5 A student carried out an experiment to determine the value of x in the formula of hydrated sodium bromide, NaBr.xH2O. Hydrated sodium bromide is heated until all the water of crystallization is removed. Anhydrous sodium bromide, NaBr, is formed. The student was given the following instructions. x Weigh a sample of the hydrated sodium bromide crystals in a pre-weighed crucible. x Heat the crucible containing the sample to remove the water of crystallization. x Allow the crucible to cool and then reweigh the crucible. The students results are shown in the table below. (a) Complete the table. Mass of crucible empty / g Mass of crucible + contents before heating / g Mass of crucible + contents after heating / g Mass of contents before heating / g Mass of contents after heating / g Mass of water removed / g 18.02 21.49 20.51 3.47 (b) (i) Calculate the number of moles of water removed on heating the hydrated sodium bromide crystals. (ii) Calculate the number of moles of anhydrous sodium bromide, NaBr, formed after heating. (2) (1) (2) *P39310A01316* 13 Turn over<br />
(iii) Use your answers from (b)(i) and (ii) to calculate the value of x. Give your answer to two significant figures. (c) Each mass reading in the table has a maximum error of 0.005 g. Calculate the percentage error in the mass of the contents of the crucible before heating the 3.47 g of crystals. (d) The correct value for x is 2. Two possible errors that might occur during the experiment are described below. x x For each error, predict the effect the error would have on the apparent mass of water removed the calculated value of (i) Carbon from the Bunsen burner flame was deposited on the crucible during heating. (2) (2) (2) Apparent mass of water removed:Value of x:14 *P39310A01416*<br />
(ii) A few crystals of hydrated sodium bromide jumped out of the crucible during heating. (2) Apparent mass of water removed:Value of x:(e) Suggest two improvements to the experiment, other than changing the balance, that would help to achieve a more accurate result. (2) First improvementSecond improvement(Total for Question 5 = 15 marks) TOTAL FOR PAPER = 50 MARKS *P39310A01516* 15<br />

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Correct Answer Reject Mark Reject Mark Reject Mark Reject Mark (Mass of contents after heating / g) 2.49 Correct Answer (Mass of water removed / g) 0.98 Correct Answer 5.44 x 10-2/ 5.4 x 10-2 (mol) 0.98 = 0.0544 / 0.054/ 18.0 IGNORE sig figs except ONE sf CQ on mass of water removed Correct Answer 2.42 x 10-2/2.4 x 10-2 (mol) 2.49 = 0.0242 / 0.024/ 102.9 NaBr molar mass = 102.9/ 23.0+79.9 (g mol-1) ALLOW 103 instead of 102.9 (1) answer CQ on molar mass IGNORE sig figs except ONE sf CQ on mass of contents after heating (1) Correct Answer (x = 0.0544/0.0242 = 2.2499) x = 2.2 to 2 sf ALLOW 2.3 answer two sig figs ALLOW 2.1,2.4,2.5 ALLOW Either numerator 0.054 or denominator 0.024 and answer correct to 2 sf (1) (1) Reject Mark (1) (1) Question Number 5(a)(i) Question Number 5(a)(ii) Question Number 5(b)(i) Question Number 5(b)(ii) Question Number 5(b)(iii)<br />
Reject Mark Correct Answer 2 x0.005 x 100% = 3.47 0.3% / 0.29% / 0.288% Ignore sf doubling the maximum error (1) answer (1) NOTE0.005 x 100% = 3.47 0.1% / 0.14% / 0.144% scores one only Reject Correct Answer (Apparent mass of water removed) decreases/less (1) (Value of x) decreases/less (than 2) Mark independently (1) Correct Answer Reject (Apparent mass of water removed) increases/more (1) (Value of x) increases/ more (than 2) (1) Mark independently Mark Mark Question Number 5(c) Question Number 5(d)(i) Question Number 5(d)(ii)<br />
Mark Reject Heat loss Correct Answer Any two improvements fromuse a lid on the crucibleheat to constant massheat with a gentle/blue flameuse a larger mass of crystalscool in a dry atmosphererepeat experimentelectrical heating IGNORE use a more accurate balance TOTAL FOR PAPER: 50 MARKS Question Number 5(e)<br />

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