2025 SAJC H2 Chem Prelim P4_Answers

Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 13 Aug 2025 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes Shift Laboratory For Examiner’s Use 1 2 3 4 Total This document consists of 21 printed pages including this page. READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, 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. The number of marks is given in the brackets [ ] at the end of each question or part question. 15 16 14 10 55

2 1. Determination of the value of x in the hydrated copper(II) sulfate, CuSO4.xH2O FA 1 is 0.150 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid. FA 3 is 1.00 mol dm–3 potassium iodide, KI. FA 4 is a solution made by dissolving 32.5 g of CuSO4.xH2O in 1.00 dm3 of solution. starch indicator In this experiment you will perform titrations to determine the value of x in the formula for hydrated copper( II) sulfate, CuSO4.xH2O, and its concentration in FA 4. You will first react a solution of Cu 2+ ions with excess iodide ions, I–. This reaction produces iodine as shown in equation 1. equation 1 2Cu2+ (aq) + 4I– (aq) → 2CuI (s) + I2 (aq) I2 has relatively low solubility in water. However, in the presence of excess I⁻, the soluble triiodide ion, I3⁻ is formed, as shown in equation 2. equation 2 I2 (aq) + I– (aq) → I3– (aq) The I3− ions formed may be titrated against a standard solution of Na2S2O3 as shown in equation 3. equation 3 I3– (aq) + 2S2O32– (aq) → 3I– (aq) + S4O62– (aq) (a) Procedure 1. Fill the burette with FA 1. 2. Pipette 25.0 cm3 of FA 4 into a 250 cm3 conical flask. 3. Use a 10 cm3 measuring cylinder to transfer 10.0 cm3 of FA 2 to the same conical flask. 4. Use a second 10 cm3 measuring cylinder to transfer 10.0 cm3 of FA 3 to the same conical flask. A white precipitate forms in a brown solution. 5. Add FA 1 from the burette into this flask. Near the end-point, when the brown solution becomes pale, add about 10 drops of starch indicator. 6. Continue adding FA 1 slowly. The end -point is reached when the solution first becomes colourless. The white precipitate remains. 7. Repeat steps 2 to 6 and step until consistent results are obtained.

3 [Turn Over Results Initial burette reading /cm3 0.00 0.00 Final burette reading /cm3 22.00 22.00 Volume of