CJC Prelim_H2_CHEM_P3_ans

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Friday 26 August 2016 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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 any four questions. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. 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 documents consists of 17printed pages and 1 blank pages. 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over ANSWER SCHEME & EXAMINERS’ COMMENTS

2 9647/03/CJC JC2 Preliminary Examination 2016 Answer any four questions. 1 Use of the Data Booklet is relevant to this question. (a) When potassium manganate(VII), KMnO4, is heated with concentrated sodium hydroxide, NaOH, bubbles of oxygen are seen and a green solution of MnO42- is obtained. The addition of barium chloride, BaC l2, to this solution precipitated out a solid X with composition by mass of 53.5% barium, 21.5% manganese and 25.0% oxygen. (i) Calculate the empirical formula of X. [1] Element Ba Mn O % mass 53.5 21.5 25.0 Ar 137 54.9 16.0 No. of moles 0.391 0.392 1.56 Mole ratio 1 1 4 Empirical formula of X is BaMnO4 (ii) When the green solution above is acidified, a brown precipitate in a purple solution is formed. Suggest the identities of the species formed upon acidification and hence state the type of reaction that has occured. [3] Brown precipitate is MnO2 Purple solution is MnO4– Disproportionation (b) A solution of acidified potassium manganate(VII), KMnO 4, can be standardi sed through titration against sodium ethanedioate, Na 2C2O4. The ethanedioate ion is oxidised to carbon dioxide in this reaction. (i) State the oxidation numbers of C in the ethanedioate ion and in carbon dioxide. [1] O.N. of C in C2O42– = +3 O.N. of C in CO2 = +4 (ii) Write the half-equation for the oxidation of the ethanedioate ion. [1] C2O42–  2CO2 + 2e– (iii) Hence, w rite the overall balanced equation for the reaction between acidified manganate(VII) and ethanedioate ions. [1] 2MnO4– + 16H+ + 5C2O42–  2Mn2+ + 10CO2 + 8H2O

3 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over (iv) In one such standardi sation procedure, 25.0 cm 3 of 0.0150 mol dm ‒3 Na2C2O4 solution required 28.85 cm3 of acidified KMnO4 to reach the end-point. Determine the concentration of KMnO 4 in the solution. You may assume that 2