Serangoon Prelim Ans

1 SSS 2024 4E Pure Chemistry Prelim Mark Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 C A C C C B D B A A 11 12 13 14 15 16 17 18 19 20 A B B B C B B A C B 21 22 23 24 25 26 27 28 29 30 D C C A A A A D A A 31 32 33 34 35 36 37 38 39 40 A D C C B B D A A C Paper 2 Section A Marker’s comments 1a A l 1 b N or O 1 c Fe 1 d Ca 1 e C 1 f O 1 2a Yellow and purple Both answers must be correct. 1 b Rf yellow dye = 0.08 (@ 64/200 x 100 = 32% ethanol) 1 c Rf value for yellow dye in pure water is 0, which means that the yellow dye cannot be separated; [1] 2

2 Rf value for red dye in pure ethanol is 0, which means that the red dye cannot be separated;. [1] *both correct but no Rf value mentioned minus 1m *1 correct but no Rf value 0m d At 80% ethanol in the solvent mixture, [1] -All 3 dyes have the same Rf values of 0.4. -All the dyes present travel the same distance/travel at the same speed /rate. -All the 3 dyes have the same solubility. Any one [1] 2 e The order and position of the dyes must be correct 1 3ai Zn + 2HCl → ZnCl2 + H2 1 aii AsH 3 / H3As 1 aiii 2

3 [1] lone (not used for bonding) pair [1] shared electrons btw As and H bi As H percentage / % 97.4 2.6 Ar 75 1 number of moles / mol 97.4/75 = 1.30 2.6/1 = 2.60 simplest ratio 1.30/1.30 = 1 2.60/1.30 = 2 Empirical formula is AsH2 [1] no. of moles expressed to 3 sf [1] ratio expressed to nearest whole no 2 bii n = 154 / 77 = 2 Molecular formula is As2H4 / H4As2 1 biii 1 4a Halogens gain (one) electrons when they react / form ions. R: require an electron 1 b At 2 (aq) Br 2 (aq) Cl2 (aq) At – (aq)   Br – (aq)   Cl – (aq)   one row / one column correct (1) all correct (2) 2 ci colourless to brown 1

4 R: yellow cii Iodide / I – ions  undergo oxidation to become I2, or  lose electrons, or  increase in oxidation state, R: Iodide/potassium iodide is oxidized (already stated in questions) Oxidising agents  gains electrons from potassium iodide  are reduced (any one) 1 1 d  particles move/diffuse from a region of higher concentration to a region of lower concentration  particles dissolve / ionise in water in the damp litmus paper to form H+ ions  The presence of the H+ ions turns the blue litmus paper to red when in contact with litmus (paper). 1 1 1 5ai Factor Ozone levels (Increase or Decrease) Smaller population density Decrease More electrical cars Decrease Sunny skies Increase 1 aii There are more vehicles / more industrial activities emitting [1] oxides of nitrogen and volatile organic compounds during mid-afternoon. Sunlight/UV is strongest [1] during mid-afternoon and will increase the rate of reaction. 2

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