2022 JPJC Prelim Paper 4 QP with Mark Schemes

© Jurong Pioneer Junior College [Turn Over NAME Mark Schemes 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 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 appropriate level of precision, in the space provided below. 6. Repeat points 2 to 5 until consistent results are obtained. Results Final burette reading / cm3 25.30 25.30 Initial burette reading / cm3 0.00 10.00 Volume of FA 2 used / cm3 25.30 25.30 ✓ ✓ [1] Gives a proper table with correct headings (‘burette’ must be stated) and units. Do not award if any final and initial burette readings are inverted. [1] Records all burette readings to the nearest 0.05 cm 3 (2 d.p.) + computes all titres correctly. Do not award if 50 is used as initial burette reading or burette reading is > 50. [1] Obtains two uncorrected titres within 0.10 cm3 + places tick (✓) or shows selected titres in exact form in the average working in (b). [3]

3 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 [Turn Over (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 = +25.30 25.30 2 = 25.30 cm3 [1] Obtains average correctly to 2.d.p. from any expts with uncorrected end-point titre values within 0.20 cm3. Do not award this mark if the titres used are not identified either in the table (by ticks) or in the calculation. Accuracy: Round any burette readings to the nearest 0.05 cm3. Check and correct subtractions in the titre table. Select the “best” titre using the hierarchy: two identical; titres within 0.05 cm3, titres within 0.10 cm3 etc. Supervisor’s average titre = 25.20 cm3 [2] within 0.2 [1] within 0.4 VFA 2 = 25.30 cm3 [3] (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] n(KMnO4) =  FA 2V3.16 158.0 1000 [1] = 3.16 25.30 158.0 1000 = 5.06  10-4 mol amount of MnO4− present = 5.06  10-4 mol [1] (ii) Calculate the amount of Fe2+ ions in 25.0 cm3 of FA 1. n(Fe2+) in 25.0 cm3 of FA 1 =  5 ans.(c)(i) [1] = 5  5.06  10-4 = 2.53  10-3 amount of Fe2+ ions = 2.53  10-3 mol [1] KMnO4 : FeSO4 1 : 5

4 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2022 (iii) Calculate the percentage by mass of iron in the sample of iron wire. [Ar: Fe, 55.8] n(Fe2+) in 500 cm3 of FA 1 = 500ans.(c)(ii) 25.0 [1] = -3 5002.53 10 25.0 = 0.0506 mol % mass of Fe = 0.0506 55.8 1003.03   = 93.2 % [1] ecf percentage by mass of iron in the wire = 93.2 % [3] [1] Shows appropriate workings for (c)(i)-(iii) + gives correct units for (b) and (c)(i)-(iii) + gives all final ans to 3sf in (c)(i)-(iii) (d) The maximum error in any single burette reading is ±0.05 cm3. (i) State the maximum possible error in the volume run from the burette recorded in any titration.  0.10 cm3 [1] with sign [1] (ii) Explain how an error of ( –0.10) cm3 may arise when using a burette to obtain an individual titre value. Initial burette reading has an error of +0.05 cm3 and the final burette reading has an error of –0.05 cm3. [1] Total error = Final – initial = (-0.05) – (+0.05) = -0.10 cm3 [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. The student was incorrect as sodium hydroxide would also react with Fe2+ in the mixture to form Fe(OH)2 ppt. [1] OWTTE Furthermore, the impurities in the wire might react with sodium hydroxide. [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) 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 care