CIE Nov 2017 v3 Paper 3 Q1

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In this experiment you will determine the value of x in the formula for hydrated copper(II) sulfate, CuSO4.xH2O. You will first react a solution of Cu2+ ions with excess iodide ions, I. This reaction produces iodine. PMT 2Cu2+(aq) + 4I(aq) 2CuI(s) + I2(aq) The amount of iodine produced can be determined by titrating with thiosulfate ions, S2O3 2. I2(aq) + 2S2O3 2(aq) 2I(aq) + S4O6 2(aq) FA 1 is 0.150 mol dm3 sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid. FA 3 is 1.00 mol dm3 potassium iodide, KI. FA 4 is a solution made by dissolving 32.5 g of CuSO4.xH2O in 1.00 dm3 of solution. starch indicator (a) MethodFill the burette with FA 1. Pipette 25.0 cm3 of FA 4 into a conical flask. Use the measuring cylinder to add 10 cm3 of FA 2 to the same conical flask. Use the measuring cylinder to add 10 cm3 of FA 3 to the same conical flask. The mixture will become brown because of the formation of I2, and will become cloudy because of the formation of the white precipitate of CuI. Add FA 1 from the burette until the mixture becomes a light brown colour. Add 10 to 20 drops of starch indicator until the mixture becomes blue-black. Continue to titrate with FA 1 until the blue-black colour disappears leaving a mixture with an off-white solid. This is the end-point. You should test that the end-point has been reached by adding 2 more drops of starch indicator. If the titration has reached the end-point the added starch indicator will cause no change in colour. 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 form below all of your burette readings and the volume of FA 1 added in each accurate titration. II III IV VI VII [7]UCLES 2017 9701/33/O/N/17<br /> (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. PMT 25.0 cm3 of FA 4 requiredcm3 of FA 1. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of thiosulfate ions in the volume of FA 1 calculated in (b). moles of S2O3 2 =mol (ii) Using the equations on page 2, calculate the number of moles of copper(II) ions in 25.0 cm3 of FA 4. (iii) Calculate the concentration, in mol dm3, of copper(II) ions in FA 4. moles of Cu2+ =mol concentration of Cu2+ in FA 4 =mol dm3 (iv) Calculate the value of x in CuSO4.xH2O.UCLES 2017 9701/33/O/N/17 x =[5] [Turn over<br />

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