CJC Prelim P2_SOL

[Turn over 9729/02/CJC JC2 Preliminary Examination 2017 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/02 Paper 2 Structured Questions 18 August 2017 2 hours Candidates answer on the Question Paper. Additional Materials: 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 all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 14 printed pages. For Examiner’s Use Paper 1 30 Paper 2 Q 1 12 Q 2 19 Q 3 18 Q 4 9 Q 5 10 Q 6 7 75 Paper 3 Q 1 22 Q 2 20 Q 3 18 Q 4 20 Q 5 20 80 Total 185 Catholic Junior College JC2 Preliminary Examinations Higher 2 Suggested Solutions

2 9729/02/CJC JC2 Preliminary Examination 2017 1 Sedimentary rocks formed at the earth's surface by the accumulation of particulate matter are usually transported to the place of deposition by water, wind, or mass movement of glaciers. Dolostone, a sedimentary rock, consists of a mixture of minerals such as calcium and magnesium carbonates. Table 1.1 below shows some properties of calcium carbonate and magnesium carbonate. Table 1.1 (a) (i) Write an expression for the solubility product, Ksp, of calcium carbonate. ................................ ................................ ................................ ...................... [1] (ii) Calculate the solubility, in mol dm-3, of calcium carbonate in water at 25 °C. [1] (iii) A saturated solution X containing calcium hydroxide and calcium carbonate has a pH of 13 at 25 °C. Given that the Ksp of calcium hydroxide is 5.5 x 10-6 mol3 dm-9 and using your answer in (a)(i), calculate the solubility of calcium carbonate in solution X. [2] Numerical values of Ksp at 25 °C Decomposition temperature / °C calcium carbonate magnesium carbonate 5.0 x 10-9 1.0 x 10-5 900 540 Ksp= [Ca2+] [CO32-] Let solubility of CaCO3 be x [Ca2+] = [CO32-] = x Ksp = [Ca2+][CO32-] x2 = 5.0 x 10-9 x = 7.07 x 10-5 mol dm-3 pOH = 14 - 13 [OH-] = 10-(14-13) = 0.100 mol dm-3 [Ca2+][OH-]2 = 5.5 x 10-6 [Ca2+] = 5.5 x 10-4 mol dm-3 [Ca2+][CO32-] = 5.0 x 10-9 [CO32-] = 5.0 x 10-9 / 5.5 x 10-4 [CO32-] = 9.09 x 10-6 mol dm-3 Since mol CaCO3 dissolved ≡ CO32– in saturated solution Solubility of CaCO3= = 9.09 x 10-6 mol dm-3

3 [Turn over 9729/02/CJC JC2 Preliminary Examination 2017 (iv) Comment on, and explain , the discrepancy in the solubi