2019 EJC Prelim PE H2 Chemistry Paper 4 (solutions _ comments for students)

© EJC 9729/04/J2PE/19 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2019 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 8 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 03 September 2019 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 paper clips, highlighters, 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 / 18 2 / 4 3 / 19 4 / 14 Total / 55 This document consists of 18 printed pages.

2 © EJC 9729/04/J2PE/19 For Examiner’s Use Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of water of crystallisation in CuSO4·nH2O Copper forms compounds containing Cu2+ or Cu+ ions. Those compounds containing Cu2+ ions tend to be relatively stable. The addition of an excess of potassium iodide, K I, to a solution of Cu 2+ ions produces iodine, I2, and a stable precipitate of Cu I. To determine the concentration of Cu 2+ via iodometric titration, it is necessary that all the Cu 2+ ions are reduced to Cu + ions. A brown suspension will be produced, which is an off -white precipitate of Cu I in a yellow -brown solution of I2. equation 1 2Cu2+ (aq) + 4I– (aq)  2CuI (s) + I2 (aq) 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 by 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– The solution should be titrated immediately after addition of K I because the I2 may be adsorbed onto the CuI precipitate, rendering the end-point less sharp. FA 1 is solid hydrated copper(II) sulfate, CuSO4·nH2O, where n is an integer. You are also provided with FA 2, 0.100 mol dm–3 sodium thiosulfate, Na2S2O3, FA 3, 1.00 mol dm–3 potassium iodide, KI, 10% potassium thiocyanate, KSCN,