2021 CJC H2 CHEM Prelim P4 solutions

1 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical August 2021 2 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and class in the boxes above. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or 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 15 and 16. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 16 printed pages and 0 blank page. [Turn over Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 / 21 2 / 18 3 / 16 Total / 55

2 Answer all the questions in the spaces provided. 1 To determine the stoichiometry of two redox reactions. FA 1 is 0.0395 mol dm–3 ammonium iron(II) sulfate, (NH4)2SO4.FeSO4.6H2O. FA 2 is 0.0150 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is 0.0250 mol dm–3 hydrogen peroxide, H2O2. You are also provided with dilute sulfuric acid, H2SO4. Potassium manganate(VII), FA 2, and hydrogen peroxide, FA 3, are both able to oxidise the Fe2+(aq) ions in acidified FA 1. MnO4–(aq) + 8H+(aq) + 5e– ® Mn2+(aq) + 4H2O(l) Fe2+(aq) ® Fe3+(aq) + e– In the presence of acid, potassium manganate(VII), FA 2, can also oxidise hydrogen peroxide, FA 3, forming oxygen gas. In this question, you will prepare five mixtures, each containing different volumes of FA 3 added to 25.0 cm3 portions of FA 1. This mixture will then be acidified by adding sulfuric acid and titrated against FA 2. A graph of the results can then be drawn. Graphical analysis of your results will enable you to determine • the volume, V, of FA 3 which reacts with 25.0 cm3 of FA 1, • the mole ratio for the reaction between Fe2+(aq) and H2O2(aq), and • the mole ratio for the reaction between MnO4–(aq) and H2O2(aq). Each titration is to be performed once only, so great care should be taken that you do not overshoot the end-points. Prepare a table in the space provided on page 4 to record, to an appropriate level of precision: • volume of FA 3 added in each mixture, VFA3, • all burette readings, • volume of FA 2 added, VFA2. volume of FA 3 / cm3 volume of FA 2 / cm3 V (graph is not to scale)

3 (a) Method (i) Perform Experiment 1 and 2 as described below. Record your results (VFA3, all burette readings and VFA2) to an appropriate level of precision, in your table. Experiment 1 1. Fill the burette labelled FA 3 with FA 3. 2. Fill the other burette with FA 2. 3. Pipette 25.0 cm3 of FA 1 into a 250 cm3 conical flask. 4. Use a measuring cylinder to add 10 cm3 of sulfuric acid into the conical flask. 5. Titrate the FA 1 solution in the conical flask with FA 2. End-point is reached when a permanent pale pink colour is obtained. Experiment 2 In this experiment, a very small volume of FA 2 will be required. 6. Repeat steps 3 to 5 but, at step 5, add 12.00 cm3 of FA 3 from the burette into the conical flask and then titrate the mixture with FA 2. (ii) The volume of FA 3 added in Experiment 2 was insufficient to react with all the Fe2+ ions in FA 1. Use your results from Experiment 1 and 2 to estimate a value for the volume of FA 3 at point V. (Sketching a graph of VFA2 against VFA3 or otherwise may help in calculating the volume of FA 3 at point V.) estimated volume of FA 3 at point V = …………………… [1] estimated vol of FA 3 = 𝟏𝟑.𝟓𝟎𝟎𝟏𝟑.𝟓𝟎&𝟓.𝟑𝟓 × 12.00 = 19.88 cm3 19.88 cm3 volume of FA 3 / cm3 volume of FA 2 / cm3 V 13.50 5.35 0 12.00

4 (iii) Experiment 3 to 5 Use your answer from (a)(ii) to select three further volumes of FA 3 in order to produce a graph of the type shown in page 1. Repeat Experiment 2 but use your selected volumes of FA 3 at step 5 and then titrate each mixture with FA 2. Results: [5] single table with appropriate headings and units all burette readings to 2 d.p. (read to 0.05 cm3) choice of one VFA3 < predicted vol at V and two VFA3 > predicted vol at V accuracy – max 2 marks Expt 1 Expt 2 Expt 3 Expt 4 Expt 5 vol. of FA 3 added, VFA3 / cm3 0.00 12.00 6.00 25.00 36.00 final burette reading / cm3 13.50 21.85 31.45 6.95 16.30 initial burette reading / cm3 0.00 16.50 22.00 4.00 6.60 vol. of FA 2 added, VFA2 / cm3 13.50 5.35 9.45 2.95 9.70

5 (b) Plot on the grid below, a graph of VFA2 on the y-axis against VFA3 on the x-axis. Draw two best-fit straight line for the point before and after V. Hence, obtain a value for the volume at V. volume of FA 3 at point V = ………………………… [5] • axes correctly labelled, with units, plus sensible linear scale with plotted points cover about half of graph grid for both axes • all points accurately plotted to within ½ small square and in correct small square • both straight lines meet at x-axis and V correctly read from graph within ½ small square • V correctly read from graph within ½ small square • all but one of the plotted points within 1 small square of best-fit line 20.00 cm3 vol of FA 2, VFA2 / cm3 2 4 6 8 10 12 14 30 vol of FA 3, VFA3 / cm3 0 5 10 15 20 25 0 35 40 (36.5, 10.0) (21.0, 0.6)

6 (c) (i) Calculate the amount of H2O2 present in V cm3 of FA 3 and hence deduce the amount of H2O2 that will react exactly with 1.00 mol of Fe2+ ions. Give your answers to three significant figures. amount of H2O2 present in V cm3 of FA 3 = …………………..… [1] amount of H2O2 that reacts exactly with 1.00 mol of Fe2+ ions = ………………..…. [1] (ii) Calculate the gradient of the line to the right of V to three significant figures, showing clearly how you did this. gradient = ............................ [2] (iii) Use the following relationship to calculate the mole ratio for the reaction between H2O2 and MnO4– ions. gradient = mol of MnO'& × [FA 3]mol of H(O( × [FA 2] mole ratio H2O2 : MnO4– = .............................. [1] amount of H2O2 = 0.0250 ´ 𝟐𝟎.𝟎𝟎𝟏𝟎𝟎𝟎 = 5.00 ´ 10–4 mol amount of Fe2+ in 25.0 cm3 of FA 1 = 0.0395 ´ 𝟐𝟓.𝟎𝟏𝟎𝟎𝟎 = 9.88 ´ 10–4 mol amount of H2O2 that react exactly with 1.00 mol of Fe2+ = 𝟓.𝟎𝟎×𝟏𝟎&𝟒𝟗.𝟖𝟖×𝟏𝟎&𝟒 = 0.506 mol 5.00 ´ 10–4 mol 0.506 mol 0.0250 ´ V𝟏𝟎𝟎𝟎 (must be 3 s.f.) above answer𝟗.𝟖𝟖×𝟏𝟎&𝟒 (must be 3 s.f.) 0.606 gradient = 𝟏𝟎.𝟎5𝟎.𝟔𝟑𝟔.𝟓5𝟐𝟏.𝟎 = 𝟗.𝟒𝟏𝟓.𝟓 = 0.606 2.75 : 1 gradient = mol of MnO𝟒& × [FA 3]mol of H𝟐O𝟐 × [FA 2] mol of H𝟐O𝟐mol of MnO𝟒& = [FA 3][FA 2] × 𝟏gradient = 0.02500.0150 × 𝟏0.606 = 2.75

7 (d) Explain, in terms of the chemistry involved, the direction of the slope of your graph (i) to the left of V …………………………………………………………………………………………… …………………………………………………………………………………………… ………………………………………………………………………………………. [1] (ii) to the right of V …………………………………………………………………………………………… …………………………………………………………………………………………… ………………………………………………………………………………………. [1] (e) A student repeated the experiment described in (a) but used 0.0395 mol dm–3 hydrogen peroxide instead of FA 3. Suggest what effect this would have on the value of V, explain your answer clearly. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………….. [1] (f) (i) One source of potential error in this experiment involves taking readings from a burette. A student carried out Experiment 2 and recorded the initial and final burette readings as 16.50 cm3 and 21.85 cm3 respectively. Calculate, to t