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 t