CIE Jun 2015 v5 Paper 3 Q1

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this experiment you will determine In FeNH4(SO4)2.xH2O, where x is the number of molecules of water of crystallisation. A known mass of this iron(III) compound reacted with excess acidi ed potassium iodide to produce iodine. You will determine the amount of iodine produced by titrating the mixture with sodium thiosulfate. iron(III) ammonium sulfate, the formula of FA 1 is 0.900 mol dm3 sodium thiosulfate, Na2S2O3. FA 2 is a solution of iodine, I2, produced as outlined in the paragraph above. starch indicator (a) Method Diluting FA 1Pipette 25.0 cm3 of FA 1 into the 250 cm3 volumetric (graduated) flask. Make the solution up to the mark using distilled water. Shake the flask to mix the solution thoroughly before using it for your titrations. Label this diluted solution of sodium thiosulfate FA 3. Rinse the pipette with distilled water. Keep FA 1 for use in Question 3. TitrationFill the burette with FA 3. Use the pipette to transfer 25.0 cm3 of FA 2 into a conical flask. Add FA 3 from the burette into the conical flask until the mixture becomes pale yellow. Then add 10 drops of starch indicator to give a blue-black colour. Continue adding FA 3 until this blue-black colour disappears. This is the end-point of the titration. Perform a rough titration and record your burette readings in the space below. The rough titre iscm3. Carry out as many accurate titrations as you think necessary to obtain consistent results. Make sure any recorded results show the precision of your practical work. Record in a suitable table below, all of your burette readings and the volume of FA 3 added in each accurate titration. II III IV VI VII [7]UCLES 2015 9701/35/M/J/15<br /> (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 requiredcm3 of FA 3. [1] (c) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Using information on page 2, calculate the concentration, in mol dm3, of sodium thiosulfate in FA 3. concentration of Na2S2O3 in FA 3 =mol dm3 (ii) Calculate the number of moles of sodium thiosulfate present in the volume of FA 3 calculated in (b). (iii) Use the equation below to calculate the number of moles of iodine that reacted with the sodium thiosulfate in (ii). moles of Na2S2O3 =mol I2 + 2Na2S2O32NaI + Na2S4O6 (iv) Calculate the concentration of I2, in mol dm3, in FA 2. moles of I2 =mol concentration of I2 =mol dm3UCLES 2015 9701/35/M/J/15 [Turn over<br /> (v) The iodine in FA 2 was produced by the reaction of iron(III) ions with excess potassium iodide. Balance the equation for this reaction.Fe3+(aq) +I(aq)Fe2+(aq) +I2(aq) Use your answer to (iv) and this equation to calculate the number of moles of iron(III) ions that reacted to produce the iodine in 1.00 dm3 of FA 2. moles of Fe3+ =mol (vi) The formula of the iron(III) compound is FeNH4(SO4)2.xH2O. 38.56 g of this compound was weighed out and added to excess aqueous acidi ed potassium iodide. FA 2 was made by making the resulting solution of iodine up to 1.00 dm3 with distilled water. Use this information and your answer to (v) to calculate the number of moles of water of crystallisation, x, in one mole of the iron(III) compound. [Ar: H, 1.0; N, 14.0; O, 16.0; S, 32.1; Fe, 55.8] x =[6] [Total: 14]UCLES 2015 9701/35/M/J/15<br />

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