XMS SEC 3 WA3 2023 MS

Xinmin Secondary School 2023 Sec 3 Weighted Assessment 3 – Answer Scheme Section A Section B Qn Suggested answers Mark 6 a Al2O3 1 b SO2 1 c Pb(NO3)2 1 d NaCl 1 [Total: 4] 7 a P: Magnesium Q: Aqueous ammonia [A: ammonium hydroxide] R: Ammonium chloride A: formulae 1 1 1 b N2 + 3H2 ⇌ 2NH3 1m for balanced chemical equation 1m for reversible arrow 2 c Nitrogen: fractional distillation of liquid air OR Hydrogen: cracking of crude oil A: Nitrogen obtained from air, hydrogen obtained from crude oil – with mention of the gas R: If no mention of gas but just the method. 1 [Total: 6] 8 ai ‘Barium meal’ contains barium sulfate which is insoluble. Hence will not produce barium ions. 1 aii Even though barium carbonate is insoluble, it will react with the acids in gastric juice (hydrochloric acid) in the stomach; [A: if lifted directly from the passage, but need to mention ‘acids in gastric juice’] to form soluble barium chloride that produces barium ions, causing poisoning. [R: if no mention of how the barium ions come about] 1 1 b Bubble the gas through limewater and a white precipitate will be formed. A: chalky, cloudy, milky 1 1 ci In a strong acid like HCl, the molecules dissociate / ionise completely in water, releasing a higher concentration of H+ ions. In a weak acid, the molecules partially dissociate / ionise in water, to give a lower concentration of H+ ions. 1 1 Question 1 2 3 4 5 Answer B A D D C

Qn Suggested answers Mark A: if answers compare ionisation of strong vs weak acid [1 m] before stating the concentration of H+ ions [1m]. cii pH pKa strong acid low low weak acid high high Every 2 correct, 1 mark d Carbonic acid is the weakest. It has the highest pKa value of 6.3 which means that it dissociates to produce the least amount of H+ ions. As a result, there would be less H + ions to react with the carbonate hence producing 0.95cm3 of CO2 gas which is the least volume. 1 1 1 e Carbon monoxide is a neutral oxide. 1 [Total: 13] 9 a 1 b Separating funnel; They are immiscible liquids. A: Fractional distillation, reason: they have different boiling points. Note: method and reason must match for full credit. 1 1 ci 4 1 cii Unlike the conventional chromatography which only uses one solvent , two-way chromatography allows for the pigments which have very similar Rf values to be further separated using a second solvent because one solvent may not be sufficient to properly separate the pigments. A: Any other possible answers which mention further separation using different solvents. 1 1 ciii Rf = 0.8/4.2 = 0.19 (2dp) A: calculations that are +/- 0.1 1 [Total: 7] water in water out