CJC Prelim P1_Worked_solutions

1 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/01 Paper 1 Multiple Choice Wednesday 29 August 2018 1 hour Additional Materials: Mult iple 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, class and NRIC/FIN number 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. 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 29 printed pages. Catholic Junior College JC 2 Preliminary Examinations Higher 2 WORKED SOLUTIONS

2 9729/01/CJC JC2 Preliminary Examination 2018 1 A sample of tungsten contains four naturally occurring isotopes, 182W, 183W, 184W and 186W. The relative atomic mass of tungsten in this sample is 183.9. What is the percentage of the isotope 182W in this sample? Isotope Relative Abundance (%) 182W ? 183W ? 184W 30.6 186W 28.6 A 10.5 C 26.4 B 14.4 D 40.8 Answer: C Let the percentage of 182W be x. The percentage of 183W would be 100 – 30.6 – 28.6 – x = 40.8 – x Thus, 183.9 = ଵ଼ଶ௫ାଵ଼ଷ(ସ଴.଼ି௫)ାଵ଼ସ(ଷ଴.଺)ାଵ଼଺(ଶ଼.଺) ଵ଴଴ x = 26.4 2 20 cm 3 of 0.100 mol dm −3 of potassium ferrate( VI), K 2FeO4, reacts with sodium ethanedioate, Na2C2O4, in an acidic medium to produce 144 cm 3 of carbon dioxide gas at room temperature and pressure. The half equation of C 2O42– is shown as follows: 2CO2 + 2e– C 2O42– What is the final oxidation state of the iron-containing species after the reaction? A +1 B +2 C +3 D +4 Answer: C Oxidation state of Fe in K 2FeO4 = +6 Amt of FeO 42– reacted = ଶ଴ ଵ଴଴଴ × 0.100 = 0.002 mol Amt of CO 2 formed = 144 ÷ 24 000 = 0.006 mol Amt of C 2O42– reacted = ½ x 0.006 = 0.003 mol

3 9729/01/CJC JC2 Preliminary Examination 2018 [Turn over Ratio of FeO 42– : C2O42– = 0.002 : 0.003 2 FeO 42– Ξ 3 C2O42– 3 mol of C 2O42– will produce 6 mol of e – and 2 mol of FeO42– will accept 6 mol of e–. Therefore, 1 mol of FeO 42– will accept 3 mol of e–. Since FeO 42– is reduced from an oxidation state of +6 to +3. 3 When attracted by a strong magnet, some species are able to exhibit paramagnetism. Such species contain unpaired electrons which are able to spin in a way which aligns parallel to the magnetic field. Which of the following species in the ground state is able to exhibit paramagnetism? 1 O 2 A l+ 3 Ti2+ 4 Cu+ A 1 and 3 only B 2 and 4 only C 1, 3 and 4