CJC 2024 Prelim H1 Chem Paper 1 Solutions

1 CHEMISTRY 8873/01 Paper 1 Multiple Choice 12 September 2024 1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, HT group and NRIC/FIN number on the Answer Sheet in the spaces provided. Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. 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 11 printed pages Catholic Junior College JC 2 Preliminary Examinations Higher 1 WORKED SOLUTIONS

2 8873/01/CJC JC2 Preliminary Examination 2024 For each question there are four possible answers, A, B, C and D. Choose the one you consider to be correct. 1 A sample of bromine contains a mixture of two isotopes, 79Br and 81Br. The relative atomic mass of Br in this sample is 79.92. What is the percentage abundance of each isotope? 79Br 81Br A 46.0% 54.0% B 41.2% 58.8% C 54.0% 46.0% D 58.8% 41.2% Concept: Mole Concept and Stoichiometry, percentage abundance Answer: C The relative atomic mass of Br is given as 79.92. As the weighted average is closer to 79 than 81, clearly there is a greater abundance of 79Br. Hence A and B are wrong. Let x be the percentage abundance of 79Br. Hence (100-x) is the abundance of 81Br. ( x 100) 79 + ( 100-x 100 ) 81 = 79.92 x = 54.0% 2 Use of Data Booklet is relevant to this question. How many atoms of silver (proton number 47) are there in a pure silver ring with a mass of 2.88 g? A 1.20 x 1021 C 1.20  1022 B 2.00 x 1021 D 1.61  1022 Concept: Mole Concept and Stoichiometry, Use of Avogadro’s number Answer: D From the Data Booklet, Ar of silver = 107.9 and Avogadro’s number = 6.02 × 1023 Hence number of atoms of silver = 2.88 ÷ 107.9 × 6.02 × 1023 = 1.61 × 1022

3 8873/01/CJC JC2 Preliminary Examination 2024 [Turn over 3 In an experiment, 100 cm3 of a 0.2 mol dm−3 solution of a metallic salt reacts exactly with 50 cm3 of 0.2 mol dm−3 aqueous sodium sulfite, Na2SO3. The half-equation for the oxidation of the sulfite ion is shown below: SO 2– 3 (aq) + H2O(l) → SO 2– 4 (aq) + 2H+(aq) + 2e− If the original oxidation number of the metal ion for the metallic salt is +3, what is the new oxidation number of the metal ion? A +1 B +2 C +4 D +5 Concept: Mole Concept and Stoichiometry, Oxidation numbers Answer: B Amt of SO32− = 𝟓𝟎 𝟏𝟎𝟎𝟎 × 𝟎. 𝟐𝟎 = 1.0× 10−2 mol Amt of e