2021 JPJC Prelim P4 Answer Scheme

¤ Jurong Pioneer Junior College [Turn Over NAME CLASS 20S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2021 CHEMISTRY (Ans Scheme) 9729/04 Higher 2 Paper 4 Practical 17 August 2021 2 hours 30 minutes Candidates answer on the Question paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number on all 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 a HB pencil for any diagrams, 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 17 and 18. Shift At the end of the examination, fasten all your work securely together. Laboratory The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 3 Total This document consists of 17 printed pages and 1 blank page.

2 ¤ Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 Answer all the questions in the spaces provided. 1 Determination of the solubility product of calcium iodate Calcium iodate, Ca(IO3)2, is a sparingly soluble salt and can dissociate in water according to the following equation. equation 1 Ca(IO3)2(s) Ca2+(aq) + 2IO3(aq) In this experiment you will determine the solubility product of calcium iodate, Ca(IO3)2. A saturated solution of calcium iodate was prepared, the concentration of iodate ions, IO3, in this saturated solution can be determined by reacting IO3 ions with an excess of I ions to form iodine, I2. equation 2 IO3– + 5I– + 6H+ → 3I2 + 3H2O The amount of iodine formed may be titrated against a standard solution of Na2S2O3 as shown in equation 3. equation 3 I2 + 2S2O32– → 2I – + S4O62– FA 1 is a filtered saturated solution of calcium iodate, Ca(IO3)2. FA 2 is an aqueous strong acid FA 3 is 0.500 mol dm3 potassium iodide, KI. FA 4 is 0.150 mol dm3 sodium thiosulfate, Na2S2O3. (a) (i) Titration of FA 1 against FA 4 1. Fill the burette with FA 4. 2. Use a pipette to transfer 25.0 cm3 of FA 1 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 2 to the conical flask. 4. Use another measuring cylinder to add 10 cm3 of FA 3 to the conical flask. The solution will turn brown as iodine is produced. 5. Run FA 4 from the burette into this flask. Near the end-point, when the yellow solution becomes pale, add about 1 cm3 of starch indicator. 6. Continue adding FA 4 slowly. The end-point is reached with the solution first becomes colourless. 7. Record your titration results, to an appropriate level of precision, in the space provided. 8. Repeat points 2 to 7 as necessary until consistent results are obtained.

3 ¤ Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over Titration results Titration number 1 2 Final burette reading / cm3 23.30 46.70 Initial burette reading / cm3 0.00 23.30 Volume of FA4 used / cm3 23.30 23.40 9 9 [3] (ii) From the titrations, obtain a suitable volume of FA 4, VFA 4, to be used in your calculations. Show clearly how you obtained this volume. VFA4 = 2 40.2330.23 = 23.35 cm3 VFA4 = 23.35 cm3 [3] (b) (i) Calculate the amount of IO3– ions present in 25.0 cm3 of FA 1. Amount of S2O32² reacted = 150.01000 35.23u = 0.00350 mol IO3² 3I2 6S2O32² Amount of IO3² reacted = 00350.06 1u = 0.000584 mol amount of IO3– ions present in 25.0 cm3 of FA 1= 0.000584 mol [2] (ii) Calculate the concentration of IO3– ions in FA 1. [IO3²] = 1000 0.25000584.0 y= 0.0234 mol dm3 concentration of IO3– ions in FA 1 = 0.0234 mol dm3 [1] (iii) Hence, determine the solubility product of calcium iodate, Ca(IO3)2. Ca(IO3)2(s) Ca2+(aq) + 2IO3(aq) Eqm conc. ½ u 0.0234 0.0234 = 0.0117 Ksp of Ca(IO3)2 = [Ca2+][IO3]2 = (0.0117)(0.0234)2 = 6.41 u 10-6 mol3dm9 solubility product of calcium iodate, Ca(IO3)2 = 6.41 u 10-6 mol3dm9 [1]

4 ¤ Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 (c) (i) The literature value of the solubility product of Ca( IO3)2 is 6.47u106 mol3 dm-9. Calculate the experimental error in your experiment by expressing the difference between your calculated solubility product in 1(b)(iii) and the literature value as a percentage of the literature value. Experimental error = 1001047.6 1041.61047.6 6 66 uu uu

= 0.927% [1] (ii) Calculate the maximum percentage error in the volume of FA 4 used in 1(a)(ii). Max. % error in volume of FA4 used = 10035.23 )05.0(2 u= r 0.428 % maximum percentage error in the volume of FA 4 used = r 0.428 % [1] (iii) A student was provided with an FA 1 solution which was not filtered properly and contained traces of undissolved Ca(IO3)2 suspended in the solution. Predict and explain how this would affect the titre obtained in her experiment. Amount of Ca(IO3)2 in the conical flask is larger, which produces more I2 that required more S2O32- to reach end point. Thus, titre value will be larger. [1]

5 ¤ Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 [Turn Over (d) Planning A student suggested that the dissolution of calcium iodate, Ca(IO3)2, shown in equation 1, is endothermic. equation 1 Ca(IO3)2(s) Ca2+(aq) + 2IO3(aq) The actual value of the enthalpy change, 'H, and the entropy change, 'S, of the reaction can be determined from the following equation. ln Ksp = S''H RT R T is the reaction temperature in Kelvin. R is the molar gas constant. The procedure you followed in 1(a)(i) can be modified to investigate the effect of temperature, T, on the solubility product of Ca(IO3)2. Plotting ln Ksp against 1 T gives a straight line of best fit. The enthalpy change, 'H, and entropy change, 'S, of the reaction can then be graphically determined. (i) Plan an investigation to determine the effect of temperature, T, on the solubility product, Ksp, of Ca(IO3)2. You many assume that you are provided with x solid calcium iodate, Ca(IO3)2. x aqueous strong acid, FA 2. x 0.500 mol dm3 potassium iodide, KI, FA 3. x 0.150 mol dm3 sodium thiosulfate, Na2S2O3, FA 4. x starch indicator x the equipment normally found in a school or college laboratory. In your plan you should include brief details of x the reactants and conditions that you would use, x how you would prepare a saturated solution, x the procedure that you would follow and the measurements that you would take, x how you would determine the Ksp of Ca(IO3)2 for each experiment.

6 ¤ Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2019 1. Use a 100 cm3 measuring cylinder to add 100 cm3 of deionised water into a 250 cm3 conical flask. 2. Leave the conical flask containing the solution to stand in a thermostatically controlled water bath, with temperature set at 30 oC. 3. Using a spatula, add a few spatulas of solid Ca(IO3)2 into the flask. Stir to dissolve all the solids using a glass rod. Keep adding more solids, with stirring after each addition, until some solids are left undissolved.