HCI Prelim P4 QP

This document consists of 18 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examination Higher 2 NAME CT GROUP 17S CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper 9729/04 24 August 2018 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an 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 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use 1 2 3 4 Deductions Total

2 © Hwa Chong Institution 2018 9729 / 04 / C2 Prelim 2018 Answer all the questions in the spaces provided. 1 To determine the concentration of potassium iodate(V) FA 1 is a solution containing potassium iodate(V), K IO3. FA 2 is 1.0 mol dm −3 sulfuric acid, H2SO4. FA 3 is 0.50 mol dm−3 potassium iodide, KI. FA 4 is approximately 0.1 mol dm−3 sodium thiosulfate, Na2S2O3. You are also provided with starch solution. You are to determine the concentration of potassium iodate(V) present in FA 1. In acidic conditions, potassium iodate(V) oxidises iodide ions to iodine. Titration of this iodine with a standard solution of sodium thiosulfate allows the concentration of potassium iodate(V) to be determined. IO 3− + 5I− + 6H+ → 3I2 + 3H2O reaction 1 I2 + 2S2O32− → 2I− + S4O62− reaction 2 The solution of FA 1 is too concentrated to titrate directly with the FA 4 solution. You will prepare a diluted solution of FA 1. You will then use FA 4 to determine the concentration of potassium iodate(V) in the diluted solution by titration, and hence determine the concentration of potassium iodate(V) in FA 1. (a) Dilution of FA 1 You will now follow the instructions given below to prepare a diluted solution of FA 1. 1. Using a burette, measure between 49.00 cm 3 and 49.50 cm 3 of FA 1 into the 250 cm 3 graduated flask. In the space below, record your burette readings and the volume of FA 1 added to the flask. Make certain that your recorded results show the precision of your working. 2. Make up the contents of the flask to the 250 cm 3 mark with deionised water. Place the stopper in the flask and mix the contents thoroughly. This solution is FA 5.

3 © Hwa Chong Institution 2018 9729 / 04 / C2 Prelim 2018 (b) Titration of FA 5 with FA 4 (i) Titration 1. Fill a burette with FA 4. 2. Pipette 25.0 cm 3 of FA 5 into a conical flask. 3. Use appropriate measuring cylinders to add to this flask • 20 cm 3 of FA 2, • 10 cm 3 of FA 3. 4. Run FA 4 from the burette into the flask until the solution becomes pale yellow. 5. Add about 10 drops of starch solution to this flask and continue adding FA 4 until the blue-black colour just disappears. 6. Record your titration results in the space below. Make certain that your recorded results show the precision of your working. 7. Repeat the titration as many times as you think necessary to obtain accurate results. Titration results 1 2 3 4 5 6 7 (ii) Mean titre From your titrations, obtain a suitable volume of FA 4 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 4 = ………………… [7]

4 © Hwa Chong Institution 2018 9729 / 04 / C2 Prelim 2018 Standardisation of FA 4 (You are not required to carry out this experiment.) The solution of sodium thiosulfate, FA 4, you are provided with is approximately 0.1 mol dm −3. The exact concentration has been determined by titrating it with a standard solution of 0.750 volume hydrogen peroxide. In acidic conditions, hydrogen peroxide oxidises iodide ions to iodine. H 2O2 + 2I− + 2H+ → I2 + 2H2O reaction 3 The liberated iodine is then titrated against FA 4. The procedure used is given below. • A burette was filled with FA 4. • 25.0 cm 3 of hydrogen peroxide was pipetted into a conical flask. • 50 cm 3 of FA 2 and 20 cm3 of FA 3 were added to the conical flask. • About 6 drops of ammonium molybdate was added as a catalyst as reaction 3 is slow. • FA 4 was added from the burette to the flask until the solution became pale yellow. • 10 drops of starch solution were added to the flask. • More FA 4 was added from the burette to the flask until the blue-black colour just disappeared. The volume of FA 4 added was 31.50 cm 3. Calculations (c) (i) Hydrogen peroxide decomposes to form water and oxygen gas only. The concentration of hydrogen peroxide solution is often expressed as a volume strength. This term refers to the volume of oxygen evolved at room temperature and pressure when the hydrogen peroxide in 1.00 cm 3 of solution is fully decomposed. For example, 10 volume hydrogen peroxide produces 10 cm 3 of oxygen gas at room temperature and pressure from 1.00 cm3 of the solution. Calculate the concentration, in mol dm −3, of hydrogen peroxide used for the standardisation of FA 4. (The molar volume of a gas at room temperature and pressure = 24.0 dm3 mol−1.) Concentration of hydrogen peroxide = ……………………… mol dm −3 [2]

5 © Hwa Chong Institution 2018 9729 / 04 / C2 Prelim 2018 (ii) Calculate the concentration, in mol dm−3, of sodium thiosulfate in FA 4. [Na 2S2O3] in FA 4 = ……………………… mol dm −3 [1] (iii) Calculate the concentration, in mol dm −3, of potassium iodate(V) in FA 5. [KIO 3] in FA 5 = ……………………… mol dm −3 [2] (iv) Calculate the concentration, in mol dm −3, of potassium iodate(V) in FA 1. [KIO 3] in FA 1 = ……………………… mol dm −3 [1] (d) The standardisation of FA 4 was repeated using hydrogen peroxide solution that was contaminated with transition metal ions, whic h act as catalysts for the decomposition of hydrogen peroxide. Deduce and explain what effect this would have had on the calculated value of the concentration of potassium iodate(V) in FA 1. Effect …………………………………………………………………………………………………… Explanation ………………………………… …………………………………………………………. …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………………………………………………………... …………………………………………………………..…………………………………….…….. [2] [Total: 15]

6 © Hwa Chong Institution 2018 9729 / 04 / C2 Prelim 2018 2 Investigation of the dissolution of potassium iodide and lithium chloride in water FA 6 is a mixture containing equal masses of potassium iodide, KI, and lithium chloride, LiCl. The change in temperature of water per gram of LiC l dissolved may be found by first determining the relationship between mass of K I dissolved in water and the change in temperature of the water due to the dissolution. The temperature change when FA 6 dissolves completely in water is then determined. (a) Dissolution of potassium iodide Different masses of K I were added separately to 25.0