NYJC Prelim 9729_P2_Answers

[Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME Teachers’ Mark Scheme CLASS TUTOR’S NAME CHEMISTRY 9729/02 Paper 2 Structured 11 September 2018 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. For Examiner’s Use 1 /24 2 /11 3 /9 4 /13 5 /18 Total /75 This document consists of 17 printed pages and 1 blank page.

2 H2 Chemistry 9729/02 NYJC J2/18 PX [Turn Over For Examiner's Use Answer all questions in the spaces provided. 1(a) An unknown sample was found to contain the anions, Cl −, ClO 3− and NO 3−. A student weighed a sample into a beaker and recorded the following data. mass of beaker and sample / g 68.962 mass of empty beaker / g 67.620 The sample was dissolved and diluted in a 250 cm 3 volumetric flask to obtain solution L. In experiment 1 , a 50 cm 3 portion of solution L was reacted with excess silver nitrate solution. The AgCl precipitated was transferred onto a dry filter paper and was placed under an infra-red lamp. The dry AgCl precipitate was weighed and the following data was obtained. Experiment 1 mass of dry filter paper and AgCl / g 0.737 mass of dry filter paper / g 0.620 In experiment 2 , a gas was bubbled into another 50 cm 3 portion of solution L to convert ClO3− to Cl− before the addition of excess silver nitrate solution. The AgCl precipitated was also dried and weighed. The following data was obtained. Experiment 2 mass of dry filter paper and AgCl / g 0.799 mass of dry filter paper / g 0.651 (i) Write the half equation for the reduction of ClO3− to Cl−. ClO3− + 6H+ + 6e−  Cl− + 3H2O [1] .......................................................................... [1] (ii) Determine the mass of Cl − in 50 cm3 of solution L. mass of AgCl precipitate in expt 1 = 0.737 – 0.620 = 0.117 g mass of Cl− present in 50 cm3 portion = [1] [1] (iii) Determine the mass of Cl − converted from ClO3− in experiment 2. mass of AgCl precipitate in expt 2 = 0.799 – 0.651 = 0.148 g mass of Cl− present in 50 cm3 portion = [1] ma