2022 JPJC Prelim Paper 4 QP

© Jurong Pioneer Junior College [Turn Over NAME CLASS 21S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2022 CHEMISTRY 9729/04 Higher 2 Paper 4 Practical 16 August 2022 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 18 and 19. 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 4 Total This document consists of 19 printed pages.

2 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of iron in a sample of iron wire For Examiner’s Use Iron wire contains impurities. You will investigate the percentage by mass of iron in a sample of iron wire. A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron(II) sulfate. Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g) You will titrate the solution of iron( II) sulfate with potassium manganate(V II) of known concentration to determine the amount of iron(II) ions present and hence the percentage by mass of iron in the wire. You may assume the impurities do not form any products that react with potassium manganate(VII). 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) FA 1 is a solution of FeSO 4 prepared by reacting 3.03 g of iron wire with sulfuric acid to make 500 cm3 of solution. FA 2 is 3.16 g dm–3 potassium manganate(VII), KMnO4. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. (a) Method 1. Fill the burette labelled “FA 2” with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Use the 50 cm3 measuring cylinder to transfer 25 cm3 of FA 3 to the same conical flask. 4. Titrate this mixture with FA 2 until a permanent pale pink colour is obtained. 5. Record your titration results, to an appro priate level of precision, in the space provided below. 6. Repeat points 2 to 5 until consistent results are obtained. Results [3] (b) From your titrations, obtain a suitable volume of FA 2, VFA 2, to be used in your calculations. Show clearly how you obtained this volume. VFA 2 = …………………….. [3]

3 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 [Turn Over (c) (i) Calculate the amount of KMnO4 present in the volume of FA 2, V FA 2, calculated in (b). [Ar: O, 16.0; K, 39.1; Mn, 54.9] For Examiner’s Use amount of MnO4− present = ……………………. [1] (ii) Calculate the amount of Fe2+ ions in 25.0 cm3 of FA 1. amount of Fe2+ ions = ……………………. [1] (iii) Calculate the percentage by mass of iron in the sample of iron wire. [Ar: Fe, 55.8] percentage by mass of iron in the wire = ……………...... [3]

4 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 (d) The maximum error in any single burette reading is ±0.05 cm3. For Examiner’s Use (i) State the maximum possible error in the volume run from the burette recorded in any titration. …………………………………………………………………………………….. [1] (ii) Explain how an error of ( –0.10) cm3 may arise when using a burette to obtain an individual titre value. …………………………………………………………………………………….. …………………………………………………………………………………….. …………………………………………………………………………………….. [1] (e) A student suggested that when a piece of iron wire was dissolved in a known volume and concentration of sulfuric acid, the number of moles of iron that reacted with the acid could be determined by working out how much acid was left after the reaction. The amount of excess acid could be determined by titrating the mixture with a known concentration of sodium hydroxide. Explain whether the student was correct. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [1] [Total: 14]

5 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 [Turn Over 2 Investigation of the kinetics of the reaction between I–(aq) and Fe3+(aq) For Examiner’s Use Iron(III) ions, Fe3+(aq), and iodide ions, I–(aq), react to give iodine, I2(aq), in the presence of acid. reaction 2 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) If acidified iron( III) chloride and potassium iodide are mixed in the presence of starch indicator and sodium thiosulfate, the iodine liberated reacts immediately with the thiosulfate ions and is reduced back to iodide ions. reaction 3 I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) When all the thiosulfate ions have reacted, the iodine which continues to be produced then turns the starch indicator blue -black. The rate of reaction 2 may be determined by timing how long it takes for the reaction to turn blue-black. You are to investigate how the rate of reaction is affected by changing the concentration of the iodide ions. FA 4 is 0.0500 mol dm–3 potassium iodide, KI(aq) FA 5 is 0.0500 mol dm–3 acidified iron(III) chloride, FeCl3(aq), FA 6 is 0.00500 mol dm–3 sodium thiosulfate, Na2S2O3(aq) FA 7 is starch indicator. Read through the whole method carefully and prepare a table for your results on page 6 before starting any practical work. (a) Method Experiment 1 1. Fill the burette labelled “FA 4” with FA 4. 2. Run 20.00 cm3 of FA 4 into the 100 cm3 beaker. 3. Use the 50 cm 3 measuring cylinder to add the following to the same 100 cm 3 beaker: ○ 20.0 cm3 of FA 6 ○ 10.0 cm3 of FA 7 4. Place the beaker on the white tile and stir the contents of the beaker using a glass rod. 5. Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 5. 6. Add this FA 5 into the same 100 cm 3 beaker and start timing immediately at the instant of mixing. 7. Stir once. The reaction mixture will turn light brown and then pale yellow before a sudden intense blue -black colouration is observed. Stop timing as soon as the solution turns blue-black. 8. Record this reaction time, t, to the nearest 0.1 s. 9. Wash out the beaker and dry it with a paper towel.

6 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 Experiment 2 1. Fill another burette with deionised water. 2. Run 7.00 cm3 of FA 4 into the 100 cm3 beaker. 3. Run 13.00 cm3 of deionised water into the beaker containing FA 4. 4. Use the 50 cm 3 measuring cylinder to add the following to the same 100 cm 3 beaker: ○ 20.0 cm3 of FA 6 ○ 10.0 cm3 of FA 7 5. Place the beaker on the white tile and stir the contents of the beaker using a glass rod. 6. Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 5. 7. Add the FA 5 to the same 100 cm 3 beaker and start timing immediately at the instant of mixing. 8. Stir once and stop timing as soon as the solution turns blue-black. Note: This may take more than 2 minutes. 9. Record this reaction time, t, to the nearest 0.1 s. 10. Wash out the beaker and dry it with a paper towe