SAJC 2022 P4 ANS

1 Name: Shift: Class: 21S Lab: ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 18 Aug 2022 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes For Examiner’s Use 1 2 3 4 Total This document consists of 22 printed pages, including 1 blank 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. 19 14 14 8 55

2 1 Determination of water of crystallisation in a sample of hydrated copper(II) sulfate The formula of hydrated copper( II) sulfate is CuSO 4•xH2O, where x refers to the number of moles of water of crystallisation. In 1(a)(i), you will perform titration to determine the value of x. Excess aqueous KI is first added to copper(II) sulfate solution. This will produce a white precipitate in a brown solution of I2. equation 1 Cu2+ (aq) + 2I–(aq) → CuI (s) + ½I2 (aq) The liberated iodine is then titrated against sodium thiosulfate. equation 2 I2 (aq) + 2S2O32– (aq) → 2I– (aq) + S4O62– (aq) You are provided with: • solid FA 1, hydrated copper(II) sulfate • FA 2, 0.100 mol dmꟷ3 sodium thiosulfate, Na2S2O3 • FA 3, potassium iodide solution, KI • Solution S, starch solution. Note: Solution S will also be used in Question 2. (a) (i) Procedure 1. Fill a burette with FA 2. 2. Weigh accurately about 5 g of FA 1. Record the mass on page 3. Transfer all the solids into a 100 cm 3 beaker. Add about 75 cm3 of deionised water and stir with a glass rod to dissolve them. 3. Transfer the solution into a 250 cm 3 volumetric flask. Rinse the beaker with deionised water and pour the washings into the volumetric flask. 4. Make up to the 250 cm3 mark with deionised water. Stopper the volumetric flask and shake well to mix. Label this solution as FA 4. 5. Pipette 25.0 cm3 of FA 4 into a conical flask and use a measuring cylinder to add 15 cm3 of FA 3 into the same conical flask. 6. Titrate this solution with FA 2 until the mixture becomes pale brown. An off-white precipitate is also present in the conical flask.

3 7. Add approximately 1 cm3 of solution S to the conical flask and continue titration until the blue-black colour just disappears, with the off-white precipitate remaining in the conical flask.