SRJC H2 Chem 2012 Prelim P1 Soln

Section A Answer all questions 1 To determine the percentage of nitrogen present in a snack, 1.0 g of the snack was boiled with concentrated sulphuric acid to convert all the nitrogen into ammonium sulphate. The ammonium salt obtained was then boiled with excess aqueous sodium hydroxide to liberate the ammonia, which was passed into 25.0 cm 3 of 0.20 mol dm –3 hydrochloric acid. The unreacted hydrochloric acid required 20.0 cm 3 of 0.10 mol dm –3 aqueous sodium hydroxide for complete neutralisation. What is the percentage by mass of nitrogen in the snack? A 2.8% B 4.2% C 7.2% D 8.4% Answer: B Amt of NH3 =( 25 1000x 0.20) – ( 20 1000x 0.10) = 0.003 mol 2NH3≡ 1(NH4)2SO4 ≡ 2N Mass of nitrogen in snack = 0.003 x 14.0 = 0.042 g % by mass = 0.042 1.0 x 100 = 4.2% 2 In an experiment, 25 .0 cm3 of 0.2 0 mol dm –3 solution of K 2AO4 reacted exactly with 25.0 cm 3 of 0.10 mol dm –3 a queous sodium sulfate(IV). The half- equation for the oxidation of the sulfate(IV) ion is shown below. SO32– (aq) + H2O (l) ® SO42– (aq) + 2H+ (aq) + 2e– Calculate the final oxidation state of A. A +2 B +3 C +4 D +5 Answer: D 2K2AO4 ≡ SO32– No. of e gained by A in K2AO4 = No. of e lost by S in SO32– = 2e 2A6+ + 2e à 2An+ 12 – 2 = 2n n = +5

3 Two elements D and E have the following properties. · D and E form ionic compounds Na2D and Na2E respectively. · Element E forms EF6 molecules whereas D is not able to do so. Which pair of electronic configurations for D and E is correct? D E A [He] 2s2 2p2 [Ne] 3s2 3p4 B [He] 2s2 2p2 [Ne] 3s2 3p2 C [He] 2s2 2p4 [Ne] 3s2 3p2 D [He] 2s2 2p4 [Ne] 3s2 3p4 Answer: D 4 The diagram below shows liquid trichloromethane and liquid benzene flowing from burettes 1 and 2 respectively. What would happen to the flow of the liquids trichloromethane and benzene when a negatively-charged rod is brought near to each of them? Liquid trichloromethane Liquid benzene A Deflected towards the rod Deflected towards the rod B Undeflected Deflected towards the rod C Deflected towards the rod Undeflected D Undeflected Undeflected Answer: C Liquid trichloromethane is polar. The partial positive charge can be attracted by the negatively charged rod. Benzene is non-polar and does not have partial charges. Burette 1 Liquid trichloromethane Burette 2 Liquid Benzene

5 The value of pV is plotted against p for two gases, G and H, where p is the pressure and V is the volume of the gas. Which of the following could be the identities of the gases? Gas G Gas H A 0.5 mol of H2 at 25 ºC 0.5 mol of H2 at 50 ºC B 0.5 mol of H2 at 25 ºC 1 mol of SO2 at 25 ºC C 0.5 mol of SO2 at 25 ºC 0.5 mol of SO2 at 50 ºC D 0.5 mol of SO2 at 25 ºC 1 mol of H2 at 25 ºC Answer: D Amount of gas H should be twice the amount of gas G. According to the shape of the curves, gas G sh