YSS 4E Pure Chem Paper 1 Prelim 2024 MS

1 Yishun Secondary School Mark Scheme Preliminary Examination 2024 Secondary 4 Pure Chemistry 6092 Paper 1 Date of exam: 28/08/2024 Setter: Widayah Answers to Paper 1 Qn Answer Explanation 1 A For the experiment, the following apparatus is needed: • 2.35 g of a salt: electronic balance • 25.0 cm3 of water: measuring cylinder • Endothermic reaction: thermometer • Stopwatch is not needed as no time interval is required 2 C Mr of: carbon monoxide, CO = 12 + 16 = 28 ethane, C2H6 = 30 helium, He = 4 sulfur dioxide, SO2 = 64 Gas which has Mr lower than air (average Mr = 28), will go through the porous pot and increase the volume in the porous pot, causing the air to leave the set -up thru the delivery tube, causing bubbles to form. 3 D • Locating agents are needed only used for the chromatography of colourless substances. They are chemicals that react with the colourless substances to form coloured spots. • As the substance at level X is travels more distance from the start line this means that it has higher Rf value and is more soluble than the substance at level Y in the solvent. 4 C At stage R to S, the temperature is decreasing towards the freezing point. Volume of liquid is decreasing as temp reaches S, the liquid turned to solid. 5 A Ne10 20 N7 15 3– O8 16 2– F9 17 ‾ no of p 10 7 8 9 no of e 10 10 10 10 no of n 10 8 8 8 6 C Mr of C2H3Br3 = 12 × 2 + 3 × 1 + 79 × 3 Mr of C2H3Br3 = 12 × 2 + 3 × 1 + 81 × 3 Mr of C2H3Br3 = 12 × 2 + 3 × 1 + 79 × 2 + 81 Mr of C2H3Br3 = 12 × 2 + 3 × 1 + 79 × 1 + 81 × 2 7 D silver chloride is insoluble in water and exist as solid at rtp. Hence, cannot conduct electricity as there is no free -moving ions when in water or at rtp. 8 A Diamond and silicon dioxide have similar tetrahedral structure. 9 A A The molecule contains a total of 10 electrons.

2 B W and X are both non-metals: Covalent bonding happens between non-metals C W is in Group 15 of the Periodic Table : W has 5 valence electrons, hence is from Grup 15 D X is hydrogen: correct 10 C Copper is a metal which conducts electricity due to the delocalised electrons. 11 B A 0.2 mol of magnesium metal, Mg = 0.2 × 6.02 × 1023 = 1.204 × 1023 B 0.3 mol of ammonia gas, NH3 = 4 × 0.3 × 6.02 × 1023 = 7.224 × 1023 C 0.4 mol of hydrogen chloride, HCl = 2 × 0.4 × 6.02 × 1023 = 4.816 × 1023 D 1.2 mol of carbon dioxide gas, CO2 = = 3 × 1.2 × 6.02 × 1023 = 2.1672× 1024 12 A SO2 H2SO4 percentage of sulfur by mass 32 32+2(16) x 100% = 50% 32 2×1+32+4(16) x 100% = 32.65% Na2S PbS2 percentage of sulfur by mass 32 2×23 +32 x 100% = 41.03 % 32 ×2 207+2(32) x 100% = 23.62% 13 B no of mole of CN = 600 24 000 = 0.025 mol Mr = 1.3 0.025 = 52 Mr of CN = 12 + 14 = 26 n = 52 26 = 2, molecular formula is B 14 B At rtp, no of mole of any gases and no of molecules are the same. mass of 1mole of any gasses is different 15 C mol of = 0.025 × 1.00 = 0.025 mol Na2SO4 :