2019 ACJC Prelim H2 Chem P1 ANS

1 © ACJC 2019 9729/01/Prelim/2019 [Turn over ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/01 Higher 2 Paper 1 Multiple Choice 18 September 2019 1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. Write your name, index number and tutorial class on the Answer Sheet in the spaces provided unless this has been done for you. There are thirty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. The use of an approved scientific calculator is expected, where appropriate. This document consists of 18 printed pages. 9729/01/Prelim/19 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2019 Department of Chemistry [Turn over

2 © ACJC 2019 9729/01/Prelim/2019 [Turn over 1 An aqueous mixture of sodium carbonate and sodium hydrogencarbonate was titrated with hydrochloric acid and the pH was recorded. What is a suitable indicator to use for detecting the first end point and the ratio of sodium carbonate to sodium hydrogencarbonate in the mixture? indicator ratio A methyl orange 1:1 B methyl orange 1:2 ✓ C thymol blue 1:1 D thymol blue 1:2 Solution The working range of methyl orange is between pH 3.1 to 4.4 and thus is not suitable for detecting the first end point between carbonate and H + at a pH that is above 7. Thus thymol blue is a more suitable choice. CO32– + H+ ⟶ HCO3– HCO3– + H+ ⟶ H2CO3 If 10 cm3 of H+ is needed to react with the carbonate ions present in solution, then the same volume of H+ is needed to react with the hydrogencarbonate produced after the first end point, i.e. 10 cm 3. Thus the other 10 cm 3 portion of H + is u sed to react with the initial hydrogencarbonate present. Hence ratio of carbonate to hydrogencarbonate is 1:1. Comment: Students who were not careful erroneously chose D because of the convenient 1:2 ratio that they see from the graph. 0 5 10 15 20 25 30 35 40 pH volume of HCl added / cm3

3 © ACJC 2019 9729/01/Prelim/2019 [Turn over 2 The most common oxidation state of americium, Am, in aqueous solution is +3. Recen