2023 SAJC H2 Prelim P4 Answers

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 14 August 2023 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes Shift Laboratory For Examiner’s Use 1 2 3 4 Total This document consists of 26 printed pages including this page. 1. Determination of x in hydrated copper( II) sulfate, CuSO 4xH2O by titrimetric 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. 14 15 13 13 55

2 [Turn Over analysis The formula of hydrated copper( II) sulfate is CuSO 4xH2O, where x represents the number of moles of water of crystallisation in one mole of the compound. You are to determine the value of x by titrating a solution of iodine, I2, with a solution of thiosulfate ions, S 2O3 2. The iodine is formed by the reaction of copper( II) ions, Cu2+, with iodide ions, I. 2Cu2+ + 4I  2CuI + I2 2S2O3 2 + I2  S4O6 2 + 2I You are provided with the following. FA 1 is hydrated copper(II) sulfate, CuSO4xH2O. FA 2 is 0.050 mol dm3 sodium thiosulfate, Na2S2O3. FA 3 is 1.0 mol dm3 potassium iodide, KI. (a) Preparation of hydrated copper(II) sulfate, FA 4 1. Weigh accurately about 3.50 g of FA 1 in a clean and dry weighing bottle . Record the mass of FA 1 used on page 4. 2. Transfer FA 1 into a 100 cm 3 beaker . Add approximately 80 cm3 of deionised water to FA 1 in the beaker and stir until the salt has dissolved. Rinse the weighing bottle with deionised water several times, adding each rinsing to the beaker. 3. Pour the contents carefully into a 250 cm 3 volumetric flask. Rinse the beaker with deionised water and add these washings to the flask. 4. Fill the flask to the mark with deionised water and shake to ensure thorough mixing. 5. Label the solution FA 4. (b) (i) Titration of iodine against FA 2

3 [Turn Over 1. Fill a burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 4 into a conical flask. 3. Use a measuring cylinder to add approximately 7.0 cm 3 of FA 3 into the same conical flask. This will form a white precipitate in a brown solution. 4. Titrate this mixture with FA 2. When the brown solution becomes pale, add 1.0 cm 3 of starch indicator. The mixture will turn a blue-black colour. 5. Continue titrating this mixture until the blue-black colour just disappears to leave behind the white precipitate. This is the end-point of the titration. 6. Repeat steps 2 to 5 until consistent results are obtained.

4 [Turn Over Results Mass of FA 1 used = 3.47 g (TARE) since solid is soluble in water Titration number 1 2 Final burette reading / cm3 28.10 28.10 Initial burette reading / cm3 0.00 0.00 Volume of FA 2 / Na2S2O3 used / cm3 28.10 28.10 Values used   [4] (ii) From your titration results, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 2 used = 28.10 + 28.10 2 = 28.10 cm3 volume of FA 2 = …………………............. cm3 [1] (c) (i) Calculate the amount, in mol, of thiosulfate ions that were present in the volume of FA 2 calculated in (b)(ii). Amount of thiosulfate ions = 0.05  28.10 1000 = 1.41  103 mol amount of S2O3 2 = ……………..................... mol [1] (ii) Use your answer to (c)(i) to obtain the amount, in mol, of copper(II) ions present in 25.0 cm3 of FA 4. Amount of Cu2+ in 25.0 cm3 of FA 4 = amount of S2O3 2 = 1.41  103 mol amount of copper(II) ions in 25.0 cm3 of FA 4 = ……............... mol [1]

5 [Turn Over (iii) Determine the concentration, in mol dm 3, of copper( II) ions in FA 4. Conc. of Cu2+ in FA 4 = 1.41×10-3 25.0×10-3 = 0.0564 mol dm3 concentration of copper(II) ions in FA 4 = ………............ mol dm–3 [1] (iv) Use your answer to (c)(iii) to calculate the Mr of CuSO4xH2O. Amount of CuSO4xH2O weighed out = amount of Cu2+ in 250 cm3 = 0.0564  250  103 = 0.0141 mol OR = 1.41  103 x 10 = 0.0141 mol Mr of CuSO4xH2O = 3.47 0.0141 = 246.1 Mr of CuSO4xH2O = ………………….......................... Hence, deduce the value of x. Show your working. [Ar: Cu, 63.5; O, 16.0; H, 1.0; S, 32.1] 63.5 + 32.1 + (4 x 16.0) + 18.0 x = 246.1  x = 4.8  5 (whole number) x = ………………….......................... [3] (d) State and explain how the use of a measuring cylinder to measure the volume of FA 3 in step 3 of (b)(i) will affect the accuracy of the titration result. [1] Since FA 3 is measured in excess, using a measuring cylinder to measure FA 3 will not affect the amount of I2 produced hence will not affect the volume / accuracy of FA 2 used. (e) (i) When titrating iodine against FA 2 , a student recorded that [1]

6 [Turn Over 23.40 cm3 of FA 2 was used. However, the actual volume of FA 2 added was 23.50 cm3. Considering that the maximum error in a single burette reading is ±0.05 cm3, explain why the student’s titre volume is accurate. The initial burette reading has an error of +0.05 cm 3 and the final burette reading has an error of –0.05 cm3. (ii) Student B suggested improving the accuracy of the titration results by changing the concentration of FA 2 to 0.040 mol dm–3. State and explain if the student ’s suggestion would make the titration results more accurate. [1] Results would be more accurate. Lower concentration of FA 2 would require larger titre volumes (with same uncertainty). Percentage error in volume measurement would be reduced. [Total: 14] 2. Determination of the kinetics of a redox reaction

7 [Turn Over In acidic solutions, iron( III) ions are reduced by iodide ions to form iron( II) ions. The iodide ions are oxidised to iodine. 2Fe3+(aq) + 2I‒(aq) → 2Fe2+(aq) + I2(aq) The rate of this reaction can be investigated by using starch indicator, whi ch turns blue-black in the presence of iodine. Sodium thiosulfate is added to the reaction mixture to react with iodine as it is formed. The blue-black colour is see n when all the thiosulfate has reacted. I2(aq) + 2S2O3 2‒(aq) → 2I‒(aq) + S4O6 2‒(aq) You will investigate how the rate of reaction is affected by changing the concentration of the iodide ions. FA 5 is 0.0500 mol dm‒3 potassium iodide, KI. FA 6 is 0.0500 mol dm‒3 acidified iron(III) chloride, FeCl3. FA 7 is 0.00500 mol dm‒3 sodium thiosulfate, Na2S2O3. (a) Prepare a table on page 10 for your results to be record ed to an appropriate level of precision . You will need to include the volume of FA 5 , volume of water, reaction time and the rate of reaction for each of the five experiments. Note: In each of these experiments, you will need to place the beaker on a printed page on page 1 of the insert. You will view the page by look ing vertically down through the mixture. You will stop the stopwatch when you can no longer see the prints on the printed page. Experiment 1 1. Fill a burette with FA 5. 2. Transfer 20.00 cm 3 of FA 5 into a 100 cm3 beaker. 3. Use a 25.0 cm 3 measuring cylinder to measure 20.0 cm 3 of FA 7 and add to th