VJC 2020 H2 Chem Prelim P3_Ans

1  VJC 2020 9729/03/PRELIM/20 [Turn over CANDIDAT E NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/03 Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 21 September 2020 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all the questions. Section B Answer one question. 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. For Examiner’s Use Section A 1 / 20 2 / 21 3 / 19 Section B 4 OR 5 / 20 Total / 80

2  VJC 2020 9729/03/PRELIM/20 This document consists of 24 printed pages and 0 blank page. Section A Answer all the questions in this section. 1 Chlorine is in Group 17 and undergoes many reactions. (a) Sketch and label, separately, the 3pz and 3𝑑𝑑2–𝑑2 orbitals present in chlorine. [2] (b) An element A reacts with chlorine to form ACl2. The electronic configuration of A2+ is [Kr]4d10. Identify the element A. [1]  Cadmium / Cd (c) The following reaction occurs when chlorine dissolves in water. Cl2(g) + H2O(l)  HClO(aq) + HCl(aq) When solutions of chlorine are used for water purification, the pH of these solutions are kept near to pH 7 by the addition of a base. Chlorine is dissolved in water to produce 1000 cm3 of a solution containing 0.170 mol of HClO and 0.170 mol of HCl. A buffer solution is then prepared by adding 0.200 mol of NaOH(s) to this solution. The NaOH reacts initially with the HCl. The remaining NaOH then reacts with HClO. (i) Write two equations to explain how this buffer is able to maintain a relatively constant pH when a few drops of H+(aq) or OH–(aq) are added to it. [1] HClO + OH–  ClO– + H2O ClO– + H+ → HClO (ii) Calculate the pH of the buffer solution. [Ka of HClO is 2.9  10–8 mol dm–3.] [1] Amount of NaOH left after reacting with HCl = 0.200 – 0.170 = 0.030 mol Amount of NaClO formed when NaOH reacts with HClO = 0.030 mol 3pz z x y

3  VJC 2020 9729/03/PRELIM/20 [Turn over Amount of HClO left = 0.170