IJC_H1_CHEM_P1_Worked_Solutions

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2  IJC 2015 Prelim 1/8872/01 [Turn over Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 An exciting chemical demonstration is the ‘barking dog’. An equation which describes the reaction is shown. 6NO + 2CS2 3N 2 + 2CO + 2SO2 + 1 4 S8 Carbon disulfide, CS2, is a liquid with a density of 1.26 g cm3. Which mass of sulfur would be formed from 5.00 cm 3 of CS2 if the reaction proceeded with 80% yield? [Mr of CS2 = 76.2; S8 = 256.8] A 1.67 g B 2.12 g C 2.65 g D 3.32 g Mass of CS2 = 1.26 x 5 = 6.30g Amount of CS2 = 6.30 / (12.0 + 32.1 + 32.1) = 0.08268 mol Amount of S = 0.08268 / 2 x ¼ = 0.01033 Mass of S formed = 0.01033 x 80/100 x 32.1 x 8 = 2.12g 2 The following half-reactions occur when potassium iodate( V), K IO3, in hydrochloric acid solution oxidises iodine to ICl2 . IO3  + 2Cl + 6H+ + 4e ICl2  + 3H2O I2 + 4Cl 2ICl2  + 2e What is the ratio of IO3  to I2 in the balanced chemical equation for the overall reaction? A 1: 1 B 1: 2 C 1:4 D 2 :1 IO3  + 2Cl + 6H+ + 4e ICl2  + 3H2O I2 + 4Cl 2ICl2  + 2e -------------------x2 IO3  + 6H+ + 2I2 + 10Cl 4ICl2  + ICl2  + 3H2O Answer = 1:2 3 Use of the Data Booklet is relevant to this question. In which pairs do both species have different number of unpaired electrons? A O and C l+

3  IJC 2015 Prelim 2/8872/01 [Turn o