2020 RI Prelim P4 Question paper

© Raffles Institution 2020 9729/04/A/20 [Turn Over CANDIDATE NAME ( ) CLASS 20S0 RAFFLES INSTITUTION 2020 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY Paper 4 Practical 9729/04 26 August 2020 2 hours 30 minutes Do not turn over the Question Booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST. Write your name and class on the space provided when instructed to do so. Give details of the practical shift and laboratory where appropriate, in the space 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 number of marks is given in brackets [ ] at the end of each question or part question. 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 19 and 20. Shift Laboratory Bench Number For Examiner’s Use Question Marks 1 / 17 2 / 15 3 / 12 4 / 11 Total / 55 This document consists of 18 printed pages and 2 blank pages.

2 © Raffles Institution 2020 9729/04/A/20 [Turn Over Answer all the questions in the spaces provided. 1 Determination of the concentration of hydrogen peroxide and the relative formula mass of hydrated halotrichite Halotrichite is a mineral which is formed by the weathering and decomposition of pyrite commonly found near volcanic vent s. Halotrichite contains Fe2+, one other cation and one anion. Both potassium manganate(VII) and hydrogen peroxide are oxidising agents which will oxidise the Fe2+ in halotrichite to Fe3+ under acidic conditions. equation 1 MnO4−(aq) + 5Fe2+(aq) + 8H+(aq) ⎯→ Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) equation 2 2Fe2+(aq) + H2O2(aq) + 2H+(aq) ⎯→ 2Fe3+(aq) + 2H2O(l) You may assume that the other cation and the anion present in halotrichite do not react with potassium manganate(VII) and hydrogen peroxide. FA 1 is a solution formed by dissolving 22.26 g of hydrated halotrichite in 100 cm3 of water. FA 2 is 0.0200 mol dm−3 potassium manganate(VII), KMnO4. FA 3 is aqueous hydrogen peroxide, H2O2. FA 4 is 0.800 mol dm–3 sulfuric acid, H2SO4. Note: FA 1 and FA 4 are also used in questions 3 and 2 respectively. In Experiment 1, you will perform the titration of FA 1 against potassium manganate( VII), FA 2. Analysis of your titration results will e nable you to determine the relative formula mass of hydrated halotrichite. In Experiment 2 , you will add different volumes of hydrogen peroxide, FA 3, to identical samples of FA 1. In each case, the amount of hydrogen peroxide you add will only partially oxidise the Fe2+ ions present. You will then complete the redox reaction of each mixture by titration with potassium manganate( VII), FA 2. Graphical analysis of your results will enable you to determine the concentration of the hydrogen peroxide in FA 3. (a) (i) Experiment 1 1. Fill a burette labelled ‘FA 2’ with FA 2. 2. Use a pipette to transfer 10.0 cm3 of FA 1 into a 100 cm3 conical flask. 3. Use a measuring cylinder to add 5 cm3 of FA 4 to the conical flask. 4. Run FA 2 from the burette into the flask. The end -point is reached w hen the solution first becomes a permanent pale pink or orange colour. 5. Record your titration results, to an appropriate level of precision, in Table 1.1. 6. Repeat points 2 to 5 until consistent titre values are obtained.

3 © Raffles Institution 2020 9729/04/A/20 [Turn Over Table 1.1 final burette reading / cm3 initial burette reading / cm3 volume of FA 2 used / cm3 [2] (ii) From your titration results, obtain a suitable volume of FA 2, V1, to be used in your calculations. Show clearly how you obtained this volume. V1 = ……………………………….. cm3 [2] (b) (i) Use your value for V1 from (a)(ii) to calculate the concentration of iron( II) ions, [Fe2+], in FA 1. [Fe2+] = ……………………………….. mol dm−3 [2] (ii) Each formula unit of the hydrated halotrichite contains only one Fe2+. Calculate the relative formula mass of the hydrated halotrichite. relative formula mass = ……………………………….. [1]

4 © Raffles Institution 2020 9729/04/A/20 [Turn Over (c) Experiment 2 Each titration is to be performed once only, so great care should be taken that you do not overshoot the end-points. 1. Fill a second burette with FA 3. 2. Use a pipette to transfer 10.0 cm3 of FA 1 into a 100 cm3 conical flask. 3. Use a measuring cylinder to add 5 cm3 of FA 4 to the conical flask. 4. Add, from the second burette, 6.00 cm3 of FA 3 to the conical flask. 5. Run FA 2 from the burette into the flask. The end-point is reached when the solution first becomes a permanent pale pink or orange colour. 6. Record your titration results, to an appropriate level of precision, in Table 1.2. 7. Repeat points 2 to 6 using two other suitable volumes of FA 3. Your selected volumes of FA 3 must not exceed 20.00 cm3 and should differ by at least 6.00 cm3. Table 1.2 volume of FA 3 added / cm3 0.00 final burette reading for FA 2 / cm3 initial burette reading for FA 2 / cm3 volume of FA 2 used / cm3  [2] enter your value for V1 from (a)(ii) in Experiment 1

5 © Raffles Institution 2020 9729/04/A/20 [Turn Over (d) Plot a graph of volume of FA 2 used, on the y-axis, against volume of FA 3 added, on the x-axis, on the grid in Fig. 1.1. Your scale on the x-axis should allow you to determine by extrapolation, V2, which is the volume of FA 3 required to react complete ly with 10.0 cm 3 of FA 1 in the absence of FA 2. Draw a best-fit straight line taking into account all of your plotted points. Fig. 1.1 [3] (e) (i) From your graph in (d), determine V2, which is the volume of FA 3 required to react completely with the Fe2+ present in 10.0 cm3 of FA 1 in the absence of FA 2. V2 = ……………………………….. cm3 [1]

6 © Raffles Institution 2020 9729/04/A/20 [Turn Over (ii) Calculate the concentration of hydrogen peroxide, [H2O2], in FA 3. [H2O2] = ……………………………….. mol dm−3 [2] (f) Sketch the graph which you plotted in (d) on the axes given below. Label the graph as (d). On the same axes, sketch the graph that would be obtained if the concentration of KMnO4 in FA 2 is higher than 0.0200 mol dm−3. Label the graph as (f). Explain your answer. ……………………………………………………………………………..…..……………….….. ……………………………………………………………………………..…..……………….….. ……………………………………………………………………………..…..……………….….. ……………………………………………………………………………..…..……………….….. ……………………………………………………………………………..…..……………….….. …………………………………………………………………………………….................... [2] [Total: 17] volume of FA 2 used / cm3 volume of FA 3 added / cm3 0

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8 © Raffles Institution 2020 9729/04/A/20 [Turn Over 2 Determination of the relative atomic mass, Ar, of M in a metal hydroxide, M(OH) x, and