2024 VJC H2 Chemistry Prelim P4 (Ans)

1 © VJC 2024 9729/04/PRELIM/24 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Paper 4 Practical Candidates answer on the Question Paper. 26 August 2024 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. 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 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 19 and 20. 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 / 12 2 / 11 3 / 19 4 / 13 Total / 55 This document consists of 18 printed pages and 2 blank pages.

2 © VJC 2024 9729/04/PRELIM/24 Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of copper in an alloy Copper forms compounds containing Cu2+ and Cu+ ions. Those compounds containing Cu2+ ions tend to be relatively stable. The addition of an excess of potassium iodide, KI, to a solution of Cu2+ ions produces iodine, I2, and a stable precipitate of Cu I as shown in equation 1 . For a titration to be accurate, it is necessary that all the Cu2+ ions are reduced to Cu+ ions. The I2 turns the solution brown. equation 1 2Cu2+ + 4I− → 2CuI + I2 I2 has a relatively low solubility in water. However, the presence of an excess of I− ions in the reaction mixture allows the soluble tri–iodide ion, I3−, to form as shown in equation 2. This ensures that the I2 formed as shown in equation 1 is fully dissolved. equation 2 I2 + I− → I3− The I3− ions formed may be titrated against a standard solution of Na 2S2O3 as shown in equation 3. equation 3 I3− + 2S2O32− → S4O62− + 3I− In 1(a), you will perform titrations to determine the percentage by mass of copper present in an alloy. FA 2 is an aqueous solution containing 11.54 g dm−3 of the alloy in acid. You are also provided with • FA 1, 0.100 mol dm−3 sodium thiosulfate, Na2S2O3, • 0.600 mol dm−3 aqueous potassium iodide, KI, • aqueous sodium carbonate, Na2CO3, • aqueous ethanoic acid, CH3CO2H, • starch indicator. The FA