2025 Prelim P3 QP_(for exchange)_H2 Chem_HCI

This document consists of 28 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examination Higher 2 CANDIDATE NAME CT GROUP 2 4 S CENTRE NUMBER S INDEX NUMBER CHEMISTRY Paper 3 Free Response 9729/03 18 September 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, CT group, centre number and index number on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE ON ANY BARCODES. Answer all questions in the spaces provided in the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. FOR EXAMINER’S USE 1 / 22 2 / 17 3 / 21 * Circle your option below * 4 / 5 / 20 Deductions (s.f.) Deductions (units) Deductions (structures) Total / 80

2 © Hwa Chong Institution 2025 9729/03/C2 Preliminary Examination DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN [Turn Over Section A Answer all questions in this section. 1 (a) Magnesium oxide, MgO, can be obtained from sea water which contains significant amounts of Mg2+ and Ca2+ ions. The steps involved are shown in Fig. 1.1. sea water containing Mg2+ and Ca2+ 1) controlled addition of CO32− filtrate addition of OH−(aq) Mg(OH)2 heat MgO 2) filter Fig. 1.1 The numerical values of the relevant solubility products, Ksp, are given in Table 1.1. Table 1.1 compound Ksp MgCO3 1.0  10−5 CaCO3 8.7  10−9 Mg(OH)2 1.1  10−11 Ca(OH)2 5.5  10−6 (i) Calculate the solubilities of MgCO3 and Mg(OH)2 respectively, in mol dm–3. [2] (ii) If the concentration of Mg2+ ions in the filtrate is 3.0  10–5 mol dm –3, calculate the concentration of OH− ions present in the filtrate when the first trace of solid Mg(OH)2 appears. [1] (iii) Explain why CO32− ions are added to the sea water before the addition of OH− ions, and why this must be controlled. [2] ……………………………………………………….………………………………..……………… ……………………………………………………….…………………………………..…………… ……………………………………………………….…………………………………..…………… ……………………………………………………….…………………………………..…………… ……………………………………………………….…………………………………..…………… ……………………………………………………….…………………………………..…………… ……………………………………………………….…………………………………..…………… ……………………………………………………….……………………………