2022 MI H2 Chem Prelim Paper 4 - Answer

Class Adm No Candidate Name: This question paper consists of 19 printed pages and 1 blank page. 2022 Preliminary Examinations Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 31 Aug 2022 2 hours 30 min Candidates answer on the Question paper. READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. 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 at the back of the Question Paper. 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. Question 1 2 3 4 Total Marks 13 17 12 13 55 Shift Laboratory

2 1 Determination of amount of water of crystallization in CuSO4·nH2O Copper is a transition metal capable of exhibiting variable oxidation states. Compounds containing Cu2+ ions tend to be relatively stable. Cu2+ ions react with excess potassium iodide, KI, to produce iodine, I2, and a stable precipitate, CuI. To determine the concentration of Cu2+ via iodometric titration, all the Cu2+ ions are reduced to Cu+ ions. A brown suspension, made up of an off -white precipitate of CuI in a brown solution of I2, will be produced. 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 Na2S2O3 as shown in equation 3. equation 3 I3– + 2S2O32– → S4O62– + 3I– The solution should be titrated immediately after addition of KI because the I2 may be adsorbed onto the CuI precipitate, rendering the end-point less sharp. You are provided with: FA 1 is solid hydrated copper(II) sulfate, CuSO4·nH2O, where n is an integer FA 2 is 0.10 mol dm−3 sodium thiosulfate, Na2S2O3 FA 3 is 1.00 mol dm−3 potassium iodide, KI FA 4 is 10% potassium thiocyanate, KSCN Starch indicator The presence of thiocyanate ion, SCN –, in the titration mixture near to the end -point will have an impact on the accuracy of the results. The procedure described is designed to improve on the accu