RI Prelims P4 Qns

- 1 - © Raffles Institution 2017 9729/A/17 [Turn Over CANDIDATE NAME ( ) CLASS 17S0 RAFFLES INSTITUTION 2017 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY Paper 4 Practical 9729/04 17 August 2017 2 hours 30 minutes Do NOT turn over the Question Booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST. Write your name and class on the space provided when instructed to do so. Give details of the practical shift and laboratory where appropriate, in the space provided. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The number of marks is given in brackets [ ] at the end of each question or part question. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 17 and 18. Shift Laboratory Bench Number For Examiner’s Use Question Marks 1 / 24 2 / 16 3 / 15 Total / 55 This Question Paper consists of 18 printed pages.

- 2 - © Raffles Institution 2017 9729/A/17 [Turn Over 1 Determination of x in hydrated copper(II) sulf ate, CuSO 4xH2O by titrimetric analysis The formula of hydrated copper (II) sulfate is CuSO4xH2O, where x represents the number of moles of water of crystallisation in one mole of the compound. You are to determine the value of x by titrating a solution of iodine, I2, with a solution of thiosulfate ions, S2O32. The iodine is formed by the reaction of copper(II) ions, Cu2+, with iodide ions, I. 2Cu2+ + 4I  2CuI + I2 2S2O32 + I2  S4O62 + 2I You are provided with the following. FA 1 is hydrated copper(II) sulfate, CuSO4xH2O. FA 2 is 0.050 mol dm3 sodium thiosulfate, Na2S2O3. FA 3 is 1.0 mol dm3 potassium iodide, KI. (a) Preparation of aqueous copper(II) sulfate, FA 4 1. Weigh accurately about 3.1 g of FA 1 in a clean and dry weighing bottle . Record the mass used in an appropriate format in the space provided on page 3. 2. Transfer the FA 1 quantitatively into a 100 cm3 beaker and dissolve it in some deionised water. Rinse the weighing bottle with deionised water several times, adding each rinsing to the beaker. Using a glass rod, stir the solution in the beaker until all the FA 1 dissolves. 3. Transfer the solution and washings from the beaker to a 250 cm 3 volumetric flask. 4. Make up the solution to the 250 cm 3 mark of the flask with deionised water, then stopper and mix thoroughly by inverting the flask a number of times. 5. Label the solution FA 4. (b) (i) Titration of iodine against FA 2 1. Fill the burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 4 into a conical flask. 3. Use a measuring cylinder to add approximately 7 cm 3 of FA 3 into the same conical flask. This will form a brown mixture consisting of iodine and an off-white precipitate. 4. Titrate this mixture with FA 2 until most of the brown has been removed, and then add approximately 5 cm 3 of starch indicator. The mixture will turn a blue-black colour.

- 3 - © Raffles Institution 2017 9729/A/17 [Turn Over 5. Continue titrating this mixture until the blue -black colour just disappears to leave behind the off -white precipitate. This is the end -point of the titration. 6. Repeat the titration as many times as necessary to provide consistent results. 7. Record titration results in the space provided below. Recorded results must show the precision of your practical work. Results [7] (ii) From your titration results, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = …………………..........................[1] (c) (i) Calculate the amount, in moles, of thiosulfate ions that were present in the volume of FA 2 calculated in (b)(ii). amount of S2O32 = …………………..........................[1]

- 4 - © Raffles Institution 2017 9729/A/17 [Turn Over (ii) Use your answer to (c)(i) to obtain the amount, in moles, of copper(II) ions present in 25.0 cm3 of FA 4. amount of Cu2+ in 25.0 cm3 of FA 4 = …………………..........................[1] (iii) Determine the concentration, in mol dm3, of copper(II) ions in FA 4. concentration of Cu2+ in FA 4 = …………………..........................[1] (iv) Use your answer to (c)(iii) to calculate the Mr of CuSO4xH2O. Mr of CuSO4xH2O = ………………….......................... Hence, deduce the value of x. Show your working. [Ar: Cu, 63.5; O, 16.0; H, 1.0; S, 32.1] x = …………………..........................[3]

- 5 - © Raffles Institution 2017 9729/A/17 [Turn Over (d) The maximum error in a single burette reading is 0.05 cm3. When titrating iodine against FA 2, a student recorded that 23.40 cm3 of FA 2 was used. However, the actual volume of FA 2 added was 23.50 cm3. Explain how the error may have arisen from using a burette to obtain an individual titre volume. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………............[1] (e) Planning When aqueous solutions of copper( II) sulfate and sodium carbonate are combined, a solid known as basic copper(II) carbonate, Cu(OH)2nCuCO3, precipitates from solution. Cu(OH)2nCuCO3 decomposes to copper(II) oxide when heated by a Bunsen flame. The percentage by mass of copper in this basic copper( II) carbonate can be determined in the laboratory using a gravimetric technique. (i) Write a chemical equation for the thermal decomposition of Cu(OH)2nCuCO3. ……………………………………………………………………………………………...[1]

- 6 - © Raffles Institution 2017 9729/A/17 [Turn Over (ii) Outline, step by step, the practical sequence for the method you would use to  carry out the weighing of a sample of Cu(OH)2nCuCO3 solid,  decompose this sample by heating, and  ensure that the decomposition is complete. You may assume that the following are available:  5 g of Cu(OH)2nCuCO3  crucible  pipe-clay triangle  weighing balance  Bunsen burner  all other common lab apparatus …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………………………………………………………………………………......... …………………………….........……………………………