Edexcel Jan 2010 Paper 2 Q19

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PMT 19 The concentration of iodine in solution can be measured by titration with sodium thiosulfate solution. I2(aq) + 2S2O3 2(aq)2I(aq) + S4O6 2(aq) (a) Name a suitable indicator which could be used for this titration. (1)(b) The amount of sulfur dioxide in the atmosphere can be measured by passing a known volume of air through iodine solution. Sulfur dioxide converts iodine to iodide ions. SO2(g) + I2(aq) + 2H2O(l)SO4 2(aq) + 4H+(aq) + 2I(aq) In an experiment, 100 m3 of air were passed through 100 cm3 of iodine, concentration 0.0100 mol dm3. The remaining iodine was titrated with sodium thiosulfate solution and reacted with 12.60 cm3 of sodium thiosulfate, concentration 0.100 mol dm3. (i) How many moles of iodine were present in the solution of the iodine at the start of the experiment? (1) (2) (2) (ii) How many moles of iodine remained in the solution at the end of the experiment? (iii) Calculate the number of moles of iodine which reacted with the sulfur dioxide, and hence the number of moles of sulfur dioxide in 100 m3 of air. 16 *N35692A01624*<br /> PMT (iv) The European Commission recommend exposure to sulfur dioxide in air should be less than 350 micrograms (350106 g) per cubic metre. Calculate whether the sulfur dioxide in this sample of air was within this limit. One mole of sulfur dioxide has mass 64.1 g. (c) Explain whether the changes below would or would not improve the experimental procedure for measuring the concentration of sulfur dioxide in air used in (b). (i) The 100 cm3 of iodine was divided into 25 cm3 samples before titration. (2) (1)(ii) The concentration of sodium thiosulfate used to titrate the iodine was changed from 0.100 mol dm3 to 0.050 mol dm3. (2)(iii) 150 m3 of air was passed through the iodine. The solutions used were of the same concentrations as in the original experiment. (2)(Total for Question 19 = 13 marks) TOTAL FOR SECTION B = 45 MARKS *N35692A01724* 17 Turn over<br />

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