2025 SAJC H2 Chem Prelim P4 Questions

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. For Examiner’s Use

3 [Turn Over Results [5] (b) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume . Volume of FA 1 = ………..………… cm3 [1] For Examiner’s Use 󠄀 M1 󠄀 M2 󠄀 M3 󠄀 M4 󠄀 M5 󠄀 M6

4 1 (c) (i) Calculate the number of moles of S2O32- in the volume of FA 1 in (b). Number of moles of S2O32- in FA 1 = ………..……………….. [1] (ii) Calculate the number of moles of copper(II) ions in 25.0 cm3 of FA 4. Number of moles of Cu2+ in FA 4 = ………..……………….. [1] (iii) Calculate the value of x in CuSO4.xH2O. [Ar: Cu, 63.5; S, 32.1; O, 16.0; H, 1.0] x = ……….........…….. [1] For Examiner’s Use 󠄀 M7 󠄀 M8 󠄀 M9

5 [Turn Over 1 (c) (iv) Blue vitriol solution is the concentrated form of FA 4. FA 4 was prepared by diluting 10.0 cm 3 of blue vitriol solution to 250 cm3 in a volumetric flask using deionised water. Hence or otherwise , determine the concentration of C u2+ in blue vitriol solution in mol dm–3. Concentration of Cu2+ in blue vitriol solution: .............. mol dm–3 [1] (d) A student performed a titration and obtained a titre volume of 30.50 cm³. Calculate the maximum percentage error in the student's titre volume. Maximum percentage error = …………………….. [1] For Examiner’s Use 󠄀 M10 󠄀 M11

6 1 (e) A student suggests that the experiment could be made more accurate if the volume of FA 3 was measured using a burette. Suggest a reason why this change would not improve the accuracy of the experiment. ……………………………………………………………………………............. ……………………………………………………………………………............. ……………………………………………………………………………............. [1] (f) A laboratory technician accidentally prepared FA 4 by dissolving 32.5 g of hydrated copper( II) sulfate in 250 cm³ of water . Suggest how this mistake would affect the volume of FA 1 required in the titration and the calculated value of x in CuSO4.xH2O. Explain your answer. ……………………………………………………………………………... ……………………………………………………………………………... ……………………………………………………………………………... ……………………………………………………………………………... [2] (g) The formation of the white Cu I precipitate can make it harder to determine the end–point of the titration. Suggest another method to measure the amount of iodine produced without using titration. ……………………………………………………………………………... ……………………………………………………………………………... ……………………………………………………………………………... [1] [Total: 15] For Examiner’s Use 󠄀 M12 󠄀 M13 󠄀 M14 󠄀 M15

7 [Turn Over 2 Determination of enthalpy change of neutralisation You will determine the concentration of sulfuric acid by react ing with a known concentration of sodium hydroxide using a thermometric method. You are to also determine the enthalpy change of neutralisation per mole of water formed when these solutions react. The equation for the reaction is shown. 2 NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2 H2O(l) FA 2 is dilute sulfuric acid, H2SO4. FA 5 is 1.90 mol dm–3 sodium hydroxide, NaOH. In this question, you will carry out a series of experiments where different volumes of FA 5 and FA 2 are mixed together. You will determine the temperature change of the mixture, T, of each experiment and then analyse your results graphically in order to determine the • concentration of H2SO4 in FA 2 • maximum temperature change, Tmax • value for the enthalpy change of neutralisation, Hneut (a) Procedure 1. Place a polystyrene cup inside a second polystyrene cup and place both cups in a glass beaker. The retort clamp provided may be used to clamp the beaker to prevent it from tipping. 2. Use a 50 cm3 measuring cylinder to transfer 50.0 cm3 of FA 5 into the polystyrene cup. 3. Place the lid with a hole in the centre on the cup and insert the thermometer through the lid. Stir the FA 5 solution gently with the thermometer. Read and record the initial temperature of the solution of FA 5 as Tinitial. 4. Place 10.0 cm3 of FA 2 into another 50 cm3 measuring cylinder. 5. Transfer the FA 2 from the measuring cylinder into the polystyrene cup and close the lid. Stir the mixture gently with the thermometer. 6. Read and record the maximum temperature of the mixture, Tmax, and the volume of FA 2 added. 7. Rinse and dry