Edexcel Jun 2017 (IAL) Paper 2 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 Tablets of potassium iodate(V), KIO3, may be used to protect against the build-up of radioactive iodine in the body. The use of potassium iodate(V) is preferred to potassium iodide because, in hot and humid conditions, the potassium iodate(V) can be stored for much longer. A very old sample of potassium iodate(V) tablets, which originally contained 85 mg of KIO3 per tablet, was analysed using the following procedure. A tablet was crushed, dissolved in deionised water and the solution and washings added to a conical flask. Then potassium iodide, KI, and hydrochloric acid, both in excess, were added to the conical flask. This mixture was titrated with 0.0600 mol dm3 sodium thiosulfate solution. This procedure was repeated and the following burette readings were obtained. Titration Final volume / cm3 19.90 Initial volume / cm3 0.00 Volume added / cm3 19.90 Mean titre / cm3 19.85 39.70 19.90 19.80 39.85 20.00 19.85 (a) State why it was not essential to carry out the third titration. (1)(b) Starch was added to the titration mixture in order to make the end-point easier to observe. (i) State the colour change observed at the end-point with starch. Fromto(ii) Identify the substance in the titration mixture that reacts with starch. (1) (1)10 *P48383A01024* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (c) The equations for the reactions involved are IO3 (aq) + 5I(aq) + 6H+(aq) (cid:111) 3I2(aq) + 3H2O(l) 2(aq) + 2I(aq) 2(aq) + I2(aq) (cid:111)(cid:3)S4O6 2S2O3 (i) Calculate the number of moles of sodium thiosulfate that reacted. (ii) Calculate the number of moles of iodine that reacted with the thiosulfate. (iii) Calculate the mass in milligrams of potassium iodate(V) in each tablet. Give your answer to three significant figures. (1) (1) (3) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48383A01124* 11 Turn over<br /> (iv) In a radiation emergency, the recommended adult dose is 170 mg of KIO3 every 24 hours. Using your result to (c)(iii), suggest whether or not the old tablets of potassium iodate(V) are suitable for use. Justify your answer. (2)(v) The experiment was repeated with a different batch of tablets. The conical flask contained 2.15104 mol of potassium iodate(V). Calculate the minimum volume of 0.100 mol dm3 hydrochloric acid that should be added to ensure that all of the potassium iodate(V) is converted to iodine and hence suggest an appropriate volume to use. (3) 12 *P48383A01224* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> A E R A S I H T N I E T I R W T O N O D (d) Potassium iodate(V) can be produced from iodine and potassium hydroxide. (i) Give the oxidation numbers of iodine in the iodine-containing species in the following equation. Hence classify the reaction. Oxidation Number 6KOH + 3I2 (cid:111) 5KI + KIO3 + 3H2O(2) Type of reaction(ii) State the conditions necessary for this reaction to occur. (1)(Total for Question 21 = 16 marks) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48383A01324* 13 Turn over<br />

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