SAJC Prelim_P1_Answers

1 [Please Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 1 Multiple Choice 17 September 2018 1 hour Additional Materials: Multiple Choice Answer Sheet, Data Booklet READ THESE INSTRUCTIONS: 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. 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 14 printed pages.

2 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 D C C B A B A D D C Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 D D D C A B C C A A Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 A D B B A B B A C A 1 To identify an oxide of nitrogen, the oxide is mixed with an excess of hydrogen and passed over a catalyst at a suitable temperature. The water collected weighs 0.133 g. The ammonia produced is neutralised by 18.50 cm3 of 0.2 mol dm-3 HCl. Which of the following could be the oxide? 1 N 2O 2 NO 2 3 NO 3 4 N 2O4 A 3 only B 1 and 2 only C 2 and 3 only D 2 and 4 only Answer: D No. of moles of water = 0.133/18 = 0.00739 No. of moles of NH3 = 18.50/1000 x 0.2 = 0.0037 Ratio of NH3 to water is 1: 2 Therefore, x : y (i.e. N to O) of oxide is also 1:2 2 This question is about sparingly soluble salts. AgCl Ag I Hg I2 Ksp 1.0 × 10−10 8.3 × 10−16 1.1 × 10−21

3 [Please Turn Over From the above information, which statements are not true? A For a solution containing 1 mol dm −3 of NaC l and NaI, the concentration of chloride ions when AgI just starts to precipitate is 1 mol dm−3. B The solubility of HgI2 is 6.50 × 10−8 mol dm−3. C The solubility of HgI2 is the lowest. D The units for Ksp for HgI2 is mol3 dm−9. Answer: C Option A: Since silver chloride and silver iodide has the same number of ions, the solubility is directly proportional to the Ksp. Hence silver iodide is precipitated before silver chloride. When silver iodide precipitates, chloride ions are still in solution and have not precipitated yet. Hence, the concentration of chloride ions is still 1 mol dm -3. Option B: Let solubility of HgI2 be x, hence Ksp = 4x3. x = 6.50 × 10−8 mol dm−3 Option C: The solubility of AgCl = 1 x 10-5 mol dm-3, AgI = 2.88 x 10-8 mol dm-3. AgI is less soluble than HgI. Option D: For HgI2 Ksp = [Hg2+][I]2, hence units is (mol dm-3)3 = mol3dm-9 3 The following tests were performed on an aqueous solution containing bromide and iodide ions. Step Test Observations 1 Add excess AgNO 3(aq). A mixture of cream and yellow precipitate were formed. 2 To the mixture from step 1,