CIE Nov 2016 v3 Paper 4 Q7

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(a) Sodium chlorate(I), NaCl O, is the active ingredient in commercial bleach. The concentration of chlorate(I) ions was determined by titration. 18 PMT 10.0 cm3 of a bleach solution was diluted to 250 cm3 in a volumetric flask using distilled water. Dilute sulfuric acid and an excess of potassium iodide solution were added to a 25.0 cm3 portion of this solution to liberate iodine. The resulting solution required 20.80 cm3 of 0.100 mol dm3 aqueous sodium thiosulfate solution to react with the iodine produced. The titration reactions are shown. Cl O + 2I + 2H+I2 + Cl+ H2O I2 + 2S2O3 22I + S4O6 2 Calculate the concentration, in mol dm3, of Cl O ions in the bleach solution. concentration of Cl O =mol dm3 [3] (b) An indicator was used in the thiosulfate-iodine titration. (i) Name a suitable indicator for this titration.[1] (ii) State the expected colour change you would observe at the end-point in this titration. fromto[1] (iii) State when in the procedure you would add the indicator.[1]UCLES 2016 9701/43/O/N/16<br /> (c) The concentration of chlorate(I) ions can also be determined by adding an excess of hydrogen peroxide to the sample of bleach and measuring the volume of oxygen gas produced. 19 PMT H2O2 + NaCl ONaCl + O2 + H2O hydrogen peroxide oxygen gas 100 cm3 measuring cylinder water 5.0 cm3 bleach When an excess of aqueous hydrogen peroxide was added to 5.0 cm3 of a different bleach solution, 82 cm3 of oxygen was produced at room temperature and pressure. Calculate the concentration of Cl O ions in this bleach solution. concentration of Cl O =mol dm3 [2] (d) Trichlorocyanuric acid, C3Cl 3N3O3, acts as a chlorine buffer and disinfectant for swimming pools. It reacts with water to give chloric(I) acid, HCl O. C3Cl 3N3O3 + 3H2O C3H3N3O3 + 3HCl O (i) Write the expression for Kc for this equilibrium. Kc = [1] (ii) In outdoor swimming pools, the HCl O is decomposed by sunlight. The decomposition of HCl O is a redox reaction which forms a gas that relights a glowing splint. Describe and explain the effect of the decomposition of HCl O on the equilibrium in (d). State the effect on Kc.effect on Kc[2]UCLES 2016 9701/43/O/N/16 [Turn over<br /> 20 (iii) The decomposition of HCl O is a redox reaction. Suggest an equation for this reaction.[1] PMT (e) The buffer solution in blood is a mixture of carbonic acid, H2CO3, and hydrogencarbonate ions, HCO3 . Healthy blood has a pH of 7.40. H2CO3 + H2O HCO3+ H3O+ Ka = 7.94107 mol dm3 (i) Explain how this buffer system acts to control the blood pH. Include equations in your answer.[2] (ii) A patients blood has a [HCO3 ] : [H2CO3] ratio of 9.5 : 1. Calculate the pH of the patients blood. pH =[2] [Total: 16] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.UCLES 2016 9701/43/O/N/16<br />

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