2022 YIJC H2 CHEM PRELIM P4 MS

1 ©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME SUGGESTED ANSWERS CLASS DATE H2 CHEMISTRY 9729/04 Paper 4 Practical Paper 23 August 2022 2 hours 30 minutes Candidates answer on question paper. Additional Materials: As listed in the Confidential Instructions Insert READ THESE INSTRUCTIONS FIRST This document consists of 24 printed pages. Write your name and class in the spaces at the top of this page. 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 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 23 and 24. 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. For Examiner’s use 1 2 3 4 Total 55 Shift Laboratory

2 ©YIJC [Turn over Answer all the questions in the spaces provided. 1 Determination of a value for the solubility product, Ksp, of calcium iodate(V), Ca(IO3)2 The solubility in water of solid calcium iodate(V), Ca(IO3)2, is low. When a sample of this salt is mixed with water, a small amount dissolves and an equilibrium between the solid salt and its aqueous ions is established. Ca(IO3)2(s) ⇌ Ca2+(aq) + 2IO3−(aq) If separate aqueous solutions containing Ca2+ ions and IO3− ions are mixed, some of the solid salt is formed and, again an equilibrium is established. When specified volumes of potassium iodate and calcium nitrate are mixed, some calcium iodate(V) is formed as a white solid. The mixture should be left to stand for some time. After the solid is removed by filtration, the amount of iodate(V) ions left in the filtrate is determined as described below. When excess potassium iodide, KI, is added to an acidified solution containing iodate(V) ions, iodine is liberated as follows. reaction 1 IO3−(aq) + 5I−(aq) + 6H+ → 3I2(aq) + 3H2O(l) The liberated iodine is then titrated with a standard solution of sodium thiosulfate. reaction 2 2S2O32−(aq) + I2(aq) → 2I−(aq) + S4O62−(aq) In this question, you will perform a titration to determine the solubility product, Ksp, of calcium iodate(V). You are provided with: • FA 1, 0.200 mol dm−3 potassium iodate, KIO3 • FA 2, 1.00 mol dm−3 calcium nitrate, Ca(NO3)2 • FA 3, 0.0400 mol dm−3 sodium thiosulfate, Na2S2O3 • FA 4, aqueous solution of potassium iodide, KI • FA 5, dilute hydrochloric acid, HCl You are also provided with starch solution.

3 ©YIJC [Turn over (a) Preparation of the reaction mixture 1. Use a measuring cylinder to transfer 50 cm3 of FA 1 to the beaker labelled reaction mixture. 2. Use a measuring cylinder to transfer 20 cm3 of FA 2 to the same beaker. 3. A precipitate will form, stir the mixture thoroughly. Leave this mixture to stand for 15 minutes to allow equilibrium to be reached. While you are waiting for the mixture to reach equilibrium, proceed with Question 2(a). (b) (i) Analysing the filtrate 1. Filter the reaction mixture through a dry filter paper into a dry conical flask, labelled FA 6. This is the filtrate, FA 6. Do not wash the white precipitate with water. 2. Fill a burette with FA 3. 3. Use a pipette to transfer 10.0 cm3 of FA 6 into a 250 cm3 conical flask. 4. Use a measuring cylinder to add about 10 cm3 of FA 4 to the conical flask. 5. Use a measuring cylinder to add about 2 cm3 of FA 5 to the conical flask. 6. Run FA 3 from the burette into the conical flask until the brown colour of the iodine fades to a pale yellow colour. 7. Add about 5 drops of starch solution to the conical flask. Continue adding FA 3 until the blue-black colour just disappears. 8. Record your titration results, to an appropriate level of precision, in the space provided below. 9. Repeat points 3 to 7 until consistent results are obtained. Rinse the conical flask thoroughly between each titration. Titration results initial burette reading / cm3 0.00 2.00 final burette reading / cm3 36.50 38.50 volume of FA 3 used / cm3 36.50 36.50 [3]

4 ©YIJC [Turn over (ii) From your titrations, obtain a suitable volume of FA 3 to be used in your calculations. Show clearly how you obtained this volume. [3] Volume of FA 3 used = (36.50 + 36.50)  2 = 36.50 cm3 (c) (i) Calculate the amount of S2O32− ions present in the volume of FA 3 recorded in (b)(ii). [1] Amount of S2O32− ions = (36.50 × 10−3) × 0.0400 = 1.46 × 10−3 mol (ii) Calculate the amount of IO3− ions present in 10.0 cm3 of the filtrate, FA 6. [1] 2S2O32−(aq) + I2(aq) → 2I−(aq) + S4O62−(aq) Amount of I2 = ½ × Amount of S2O32− = ½ × (1.46 × 10−3) = 7.30 × 10−4 mol IO3−(aq) + 5I−(aq) + 6H+ → 3I2(aq) + 3H2O(l) Amount of IO3− in 10.0 cm3 = 1/3 × Amount of I2 = 1/3 × 7.30 × 10−4 = 2.4333 × 10−4 mol (iii) Calculate the total amount of IO3− ions present in the filtrate, FA 6. [1] Amount of IO3− in 70.0 cm3 = (2.4333 × 10−4) × 7 = 1.70 × 10−3 mol (d) (i) Using the initial amount of IO3− ions in the reaction mixture prepared in (a), and your answer from (c)(iii), calculate the amount of IO3− ions precipitated as Ca(IO3)2. [1] Initial amount of IO3− ions in the reaction mixture prepared = 0.200 × 50.0 × 10−3 = 0.0100 mol Amount of IO3− precipitated = 0.0100 – 1.70 × 10−3 = 8.30 × 10−3 mol

5 ©YIJC [Turn over (ii) Deduce the amount of Ca2+ ions removed by precipitation in (a), point 3, and hence, calculate the amount of Ca2+ ions left in FA 6. [2] Ca(IO3)2(s) ⇌ Ca2+(aq) + 2IO3−(aq) Amount of Ca2+ removed by precipitation = ½ × Amount of IO3− precipitated = ½ × 8.2966 × 10−3 mol = 4.15 × 10−3 mol Initial amount of Ca2+ added = 1.00 × 20.0 × 10−3 = 0.0200 mol Amount of Ca2+ left in FA 6 = 0.0200 – (4.15 × 10−3) = 0.0159 mol (e) (i) Write an expression for the solubility product, Ksp, of calcium iodate(V). [1] Ksp = [Ca2+][ IO3−]2 (ii) Use this expression, together with your answers to parts (c)(iii) and (d)(ii) to calculate a value for this solubility product. Include units in your answer. [1] [Ca2+] = 0.0159  (70  1000) = 0.227 mol dm−3 [IO3−] = (1.70 × 10−3)  (70  1000) = 0.024285 mol dm−3 Ksp = 0.227 × (0.024285)2 = 1.34 × 10−4 mol3 dm−9 (f) A student added solid calcium nitrate to his filtrate, FA 6. Predict, qualitatively, the effect of such an addition on the filtrate, and on the magnitude of the mean titre, in part (b)(ii). Explain your answer. [2] Predictions: More precipitate will form and the titre value will be lower. Explanation: The addition of Ca(NO 3)2 increases the concentration of Ca 2