SMSS Sec 3 pure chem answer key 2024

1 2024 SMSS Sec 3 Chemistry EOY Exam Marking Scheme Paper 1 MCQ (20 marks) Question 1 2 3 4 5 6 7 8 9 10 Answer C A B B B A C A B D Question 11 12 13 14 15 16 17 18 19 20 Answer B B C D C B D D D A Paper 2 (65 marks) No Answers marks 1 (a) carbon monoxide (b) water / carbon monoxide (c) copper(II) oxide and sulfuric acid/hydrochloric acid (d) lithium and bromine/iodine (e) silicon dioxide / methane / silicon (Any two) A: answer using formulae/symbols, provided correct e.g. I2 and Br2) [1] [1] [1] [1] [1] 2 (a) Blue dye and yellow dyes have Rf value of 0 (or *almost 0). No or *little separation of the two dyes occurred. * ECF allowed (b) 64% of ethanol and 36% of water (c) Rf value of blue dye is 0.7 (no units) Let distance travelled by blue dye be x cm3 x ÷ 10.0 = 0.7 x = 7.0 Distance travelled by blue dye is 7.0 cm3 (to 1 d.p.) [1] [1] [1] [1] [1] 3 (a) Same number of protons but different number of neutrons. Both have 92 protons. Uranium-235 has 143 neutrons while uranium-238 has 146 neutrons Accept: same proton number but different nucleon number (b) Average atomic mass = (0.7 X 235)+(99.3 X 238) 100 = 238 (to 3 s.f.) no units (c) Helium / He [1] [1] [1] [1] [1] [1] Penalise max of 1 m for wrong units and sign figs / dec pl for Q2 and Q3

2 4(a) [1] [1] 4(b) (i) False (ii) True (iii) False [1] [1] [1] 5 (a) [1] [1] 5(b) (i) NaH + H2O → NaOH + H2 (ii) green to purple/violet (Reject: blue) (iii) No. of mole of NaH = 0.24 24 = 0.01 No. of mole of H2O = 1.8 18 = 0.1 (1 mol of NaH react with 1 mol of H2O) Thus NaH is the limiting reactant No. of mole of NaOH formed = 0.01 Mass of NaOH = 0.01 X 40 g = 0.400 g (to 3 s.f.) Note: No ECF within parts [1] [1] [1] [1] [1] 6(a) Pure ethanedioic acid does not contain mobile charged particles / ions / contain only molecules with no charge. In water, the acid dissociates/ionizes to form mobile ions which are charge carriers / conduct electricity. Idea that mobile ions are present in aqueous acid but not pure acid. Idea that dissociation of acids to form ions occur only when there is water. [1] [1] correct shared electron pairs correct unshared electrons correct charges correct electronic arrangement No full marks if only valence e are shown One or/and both steps Award 2 m if answers are T, F, T

3 6(b) (i) From the graph, volume of HCl needed is 25.00 cm3 No. of mole of NaOH = 0.200 mol/dm3 X 0.02 dm3 = 0.004 mol No. of mole of HCl reacted = 0.004 mol Concentration of HCl = (0.004 ÷ 0.025) = 0.160 mol/dm3 (to 3 s.f.) (ii) pH 9 (ii) bromothymol blue and phenolphthalein [1] [1] [1] [1] Penalise max of 1 m for wrong units and sign figs / dec pl for Q5(b) and Q6(b) 7(a) Add water to s