BTYSS 2024 FYE 4E CHEM MS

Answers Paper 1 1 2 3 4 5 6 7 8 9 10 D C B B D C B C B A 11 12 13 14 15 16 17 18 19 20 B C B B D A B C D C 21 22 23 24 25 26 27 28 29 30 D A D D D D C D A B 31 32 33 34 35 36 37 38 39 40 C D B C C B C A D A Paper 2 Section A Question no Answers Marks 1 (a) PbO 1 (b) KI 1 (c) SO2 1 (d) CuCO3 1 (e) PbBr2 1 Total:5 Remarks/ common mistakes: - If students wrote 2 answers, only the first one will be marked. - For (b), many students wrote PbBr2 which is incorrect as it is insoluble - For (e), many students wrote KI and it is incorrect as KI is a soluble salt and does not prepared using the precipitation method. 2(a) 76/100 x 35 + 24/100 x 37 = 35.5 Accept: 35.48. No mark is penalised for wrong units. 1 (b)(i) Simple (covalent) molecular structure./Simple covalent molecule/ simple covalent structure with small discrete molecules. It has a low melting and boiling point as it is made up of weak intermolecular forces of attraction between the molecules where little energy is required to overcome the forces of attraction. Reject: simple covalent bond/ simple covalent structure. Remarks: Most students unable to state the structure present. Some students confused with ionic bonding or giant covalent structure. 1 1

(b)(ii) 2 Remarks: Some students draw ionic bonding instead. Students did badly for this question, Total:5 3(a)(i) It indicates that chlorine has an oxidation state of +5. Badly done. Reject: Charge of chlorine is +5, valence electrons are 5, just state the (V) represent oxidation state but did not indicate value. 1 (a)(ii) The statement is true. As the oxidation state of chlorine in NaClO3 is decreased from +5 to −1 in NaCl, chlorine is being reduced. As the oxidation state of oxygen in NaClO3 is increased from -2 to 0 in O2, oxygen is being oxidised. As both reduction and oxidation is happening simultaneously, it is a redox reaction. Remarks: Many students compared the oxidation state of sodium instead of chlorine. Stated the oxidation state of oxygen as -6 instead of -2. 1 1 (a)(iii) Use a magnet to attract the iron catalyst from the mixture and a pure and dry sample of sodium chloride will be obtained. Accept: using magnetic attraction /magnetic separation to separate iron form sodium chloride. 1 (b) Sodium chlorate(V) will decompose before its boiling point can be measured. Reject: sodium chlorate (V) sublimes/temperature is too high to be measured. 1 (c) Number of moles of O2 = 450 /24000 = 0.01875mol From equation, 3 mol O2 : 2 mol NaClO3 0.01875 mol O2 : 0.0125 mol NaClO3 1 T T Cl Cl

Mr of NaClO3 = 106.5 Mass of NaClO3 = 106.5 x 0.0125 = 1.33g Percentage purity = (1.33/5) *100 = 26.6% (R: if round off to 1dp, A: 26.625%) Accept: No of moles of NaClO3 = 5/106.5 = 0.0469mol Volume of O2 (theoretical) = 0.0469 /2 x3 x24 = 1.69dm3 Percentage purity = 450/1690 x100 = 26.6% 1