2020 SAJC H2 Prelim Paper 1 (Answers)

1 [Turn over CANDIDATE NAME CLASS 19S ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination CHEMISTRY Paper 1 Multiple Choice 9729/01 September 2020 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 and class on the Answer Sheet in the spaces provided. 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. 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 15 printed pages.

2 [Turn over 1 Carbon disulfide, CS2, burns in oxygen according to the following equation. CS2(g) + 3O2(g)  CO2(g) + 2SO2(g) 10 cm3 of carbon disulfide was completely burnt in 50 cm3 of oxygen. The resulting mixture was then treated with excess of aqueous sodium hydroxide. All measurements of volume were made at the same temperature and pressure in which all the reactant and products were in the gaseous state. What was the volume of the gas after burning and after adding NaOH (aq)? volume of gas after burning / cm3 volume of gas after adding NaOH(aq) / cm3 A 30 10 B 30 20 C 50 20 D 50 40 Ans: C Vol of excess O2 = 20 cm3 Volume of gas produced after burning = 20 + 10 + 20 = 50 cm3 CO2 and SO2 are both acidic gases that reacts with aq NaOH. vol of gas after adding aq NaOH = 50 – 30 = 20 cm3

3 [Turn over 2 1 mole of X2O32− oxidised 68.1 dm3 of sulfur dioxide gas to sulfate ions at standard temperature and pressure. What is the final oxidation state of X? A −2 B −1 C +1 D +2 Ans: B n(SO2) = 68.1/22.7= 3 SO2 + 2H2O  SO42- + 4H+ + 2e X2O32− : SO2 : e 1 : 3 : 6 1 mol of X2O32− accepts 6 mol of e 1 mol of X accepts 3 mol of e Initial OS of X is + 2, final OS of X is -1

4 [Turn over 3 Use of the Data Booklet is relevant to this question. Which species have the same number of unpaired electrons as chlorine atom? 1 Li+ 2 Mg+ 3 O− 4 F− A 2 and 3 B 3 and 4 C 1 only D 2 only Ans: A Chlorine has only one unpaired electron. Li+: 1s2 Mg+: [Ne]3s1 O−: [He] 1s2 2s22p5 F-: [Ne]

5 [Turn over 4 Hospitals require large amount of oxygen gas in the treatment of patients with respiratory symptoms. Oxygen is liquefied under pressure and stored in special cylinders. It vapourises when pressure is released. Which statement best describes why oxygen gas liquefies under pressure? A High pressure lowers the kinetic energy of oxygen molecules, creating less disordered liquid state. B High pressure pushes the molecules close