CJC Prelim P4 Answers

1 [Turn over 9729/04/CJC JC2 Preliminary Examination 2018 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical Thursday 16 August 2018 2 hours 30 minutes Candidates answer on the Question Paper READ THESE INSTRUCTIONS FIRST Write your class and name 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 you working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 13 and 14. 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. This document consists of 14 printed pages. Catholic Junior College JC 2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 17 2 18 3 20 Total 55 WORKED SOLUTIONS

2 9729/04/CJC JC2 Preliminary Examination 2018 1 Z is an oxidising agent. You are required to use a solution of iodine in potassium iodide to determine the mass of Z needed to liberate 1 mole of iodine from potassium iodide. FA3 is an aqueous solution of iodine, I2, in potassium iodide, KI. FA4 is a 0.100 mol dm–3 aqueous sodium thiosulfate, Na2S2O3.5H2O. FA5 is an aqueous solution containing 2.00 g dm–3 Z, an oxidising agent. You are to follow the following procedure and record your results as instructed. Titration 1 1 By using a pipette, transfer 25.0 cm 3 of FA3, an aqueous solution of iodine in potassium iodide, into a conical flask. 2 Titrate the iodine present with FA4, 0.100 mol dm–3 aqueous sodium thiosulfate, from a burette until the solution becomes colourless. 3 Repeat the titration as many times as you think necessary to obtain accurate results. 4 Record your results of titration in a suitable tabulated form below. Titration of iodine in FA3 with FA4 Rough Accurate Final burette reading /cm3 19.90 39.60 19.70 Initial burette reading /cm3 0.00 19.90 0.00 Volume of FA4 used /cm3 19.90 19.70 19.70 √ √ Summary 25.0 cm3 of FA3 required 19.70 cm3 of FA4 for titration. Show which results you used to obtain this value of the volume of FA4 by placing a tick ( √) under the readings used. [5] At least 2 sets of burette readings taken. Burette readings recorded to 2 d.p. Precision of V FA4 ± 0.10 cm3 and correct average of VFA4 taken. (a) Calculate how many moles of sodium thiosulfate were contained in the volume of FA4 found necessary to react with 25.0 cm3 of FA3. Amount of Na2S2O3 = ( ૚ૢ.ૠ૙ ૚૙૙૙ x 0.100) mol = 1.97 x 10-3 mol Moles of Na2S2O3 = 1.97 x 10-3 [1] (b) Calculate how many moles of I2 present in 25.0 cm 3 of FA3 using the following equation for the reaction between iodine and thiosulfate ions. I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) Amount of I2 = ( ૚ ૛ x 1.97 x 10-3) mol = 9.85 x 10-4 mol Moles of I2 = 1.97 x 10-3 [1]

3 [Turn over 9729/04/CJC JC2 Preliminary Examination 2018 Titration 2 1 By using a pipette, transfer 25.0 cm 3 of FA3, an aqueous solution of iodine in potassium iodide, into a conical flask and then using a second pipette, add 25.0 cm 3 of FA5, an aqueous solution containing 2.00 g dm-3 of the oxidising agent, Z. 2 Titrate the total iodine present with FA4, 0.100 mol dm-3 aqueous sodium thiosulfate, from a burette as before until the solution becomes colourless. 3 Repeat the titration as many times as you think necessary to obtain accurate results. 4 Record your results of titration in a suitable tabulated form below. Titration of total iodine now in FA3 with FA4 Rough Accurate Final burette reading /cm3 29.70 29.50 29.50 Initial burette reading /cm3 0.00 0.00 0.00 Volume of FA4 used /cm3 29.70 29.50 29.50 √ √ Summary 25.0 cm3 of FA3 to which 25.0 cm 3 of FA5 had been added required 29.50 cm3 of FA4 for titration. Show which results you used to obtain this value of the volume of FA4 by placing a tick ( √) under the readings used. [3] Precision of V FA4 ± 0.10 cm3 and correct average of VFA4 taken. Accuracy of titration results. (c) Calculate how many moles of sodium thiosulfate were contained in the volume of FA4 found necessary to react with the total iodine in the flask after 25.0 cm3 of FA3 and 25.0 cm3 of FA5 had been mixed. Amount of Na2S2O3 = ( ૛ૢ.૞૙ ૚૙૙૙ x 0.100) mol = 2.95 x 10-3 mol Moles of Na2S2O3 = 2.95 x 10-3 [1] (d) Calculate how many moles of I2 present in the flask after 25.0 cm3 of FA3 and 25.0 cm3 of FA5 had been mixed. I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq Amount of I2 = ( ૚ ૛ x 2.95 x 10-3) mol = 1.475 x 10-3 mol Moles of I2 = 1.475 x 10-3 [1] (e) Calculate how many moles of I2 were liberated from potassium iodide in the flask by 25.0 cm3 of FA5.

4 9729/04/CJC JC2 Preliminary Examination 2018 Amount of I2 liberated = (1.475 x 10-3 - 9.85 x 10-4) mol = 4.90 x 10-4 mol Moles of I2 = 4.90 x 10-3 [2] (f) Calculate the mass of Z present in 25.0 cm3 of FA5. Mass of Z = ( ૛૞.૙ ૚૙૙૙ x 2.00) g = 0.0500 g Mass of Z = 0.0500 g [1] (g) Hence, from (e) and (f), calculate the mass of Z needed to liberate 1 mole of I2. Mass of Z needed to liberate 1 mol of I2 = ( ૙.૙૞૙૙ ૚૙ష૝ ) g = 102 g Mass of Z = 102 g [2] [Total: