NJC_H1_CHEM_P1_answers

1 NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 1 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 1 Multiple Choice Additional Materials: Multiple Choice Answer Sheet Data Booklet 8872/01 Thursday 17 Sept 2015 50 minutes READ THESE INSTRUCTIONS FIRST Write in soft pencil. Write your name, subject class and registration number on the Answer Sheet in the spaces provided and shade the appropriate boxes for your registration number. There are thirty questions in 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 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 paper consists of 10 printed pages including this cover page.

2 Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1. Use of the Data Booklet is relevant to this question. Element X forms ion X2+ with 4 unpaired electrons. What could be element X? You’ll need to do trial and error for this question by writing out the valence electronic configuration of X2+ for each option below. A Mg2+: 2s2 2p6  no unpaired electrons B Cr2+: 3d4  4 unpaired d electrons C Ti2+: 4s2  no unpaired electrons D Ni2+:3d7  3 unpaired d electrons Answer: B 2. A solution of Sn 2+ ions can reduce an acidified solution of MnO 4 – ions. The Sn 2+ ions are oxidised to Sn4+ ions in this reaction. How many moles of MnO4 – ions are required to react completely with a solution containing 9.5 g of SnCl2 (Mr = 190)? Sn2+  Sn4+ + 2e x 5 MnO4 – + 8H+ + 5e  Mn2+ + 4H2O x 2 No. of moles of SnCl2 = 190 5.9 = 0.05 mol No. of moles of KMnO4 = 0.05 x 5 2 = 0.02 mol Answer: B 3. Use of the Data Booklet is relevant to this question. Magnesium nitrate, Mg(NO3)2, will decompose when heated to give a white solid, MgO, and a mixture of gases. One of the gas is an oxide of nitrogen. 7.4 g of anhydrous magnesium nitrate is heated until no further reaction takes place. What is the mass of the white solid formed? No. of moles of Mg(NO3)2 = 3.148 4.7 = 0.0499 mol Since 1 mole of Mg(NO3)2 will decompose to give 1 mole of MgO, No. of moles of MgO = 0.0499 mol Mass of MgO = 0.0499 x 40.3 = 2.01 g Answer: B

3 4. AlCl3 vapour forms molecules with formula Al2Cl6 as it is cooled. What happens to the bond angles during the change from AlCl3 to Al2Cl6? In AlCl3, there are 3 bond pairs and 0 lone pairs around Al, the shape is trigonal planar about Al and bond angle is 120⁰. In Al2Cl6, there are 4 bond pairs and 0 lone pairs around Al, the shape is tetrahedral