NYGH 2017-S3EOY-IP Chem P1 and P2 Ans

Secondary 3 (IP) EOY Answers 2017 Sec 3 End-of-Year Exam Chemistry Answers Paper 1 1. B 2. D 3. D 4. C 5. A 6. B 7. B 8. C 9. B 10. C 11. C 12. B 13. C 14. D 15. B 16. A 17. B 18. C 19. A 20 B 21. B 22. B 23. A 24. D 25. C 26. D 27. B 28. A 29. A 30 B Paper 2 Question Answers Marks A1 (a) Spot A is more soluble in water than spot B while Spot C is not/least soluble in water. [1] (b) He would observe that the boiling occurred over a range of temperatures [1], because the colourless liquid was impure/a mixture [1]. Not accepted: The liquid contained several substances (vague)/the boiling point was higher (than the b.p. of which component? This is impossible to ascertain) [2] Total [3] A2 (a) protons: 16 valence electrons: 6 [2] (b)(i) Not penalized although not recommended: -Brackets for calcium/no brackets for sulfide -not pairing of “dot” with “cross” for transferred electrons [1] (b)(ii) No. Calcium sulfide has a giant ionic crystal lattice [1] at room temperature. Its ions are held in fixed positions/are not mobile [1]. Note: Penalty for any contradictory terms e.g. use of “atoms”/”molecules” interchangeably with “ions”. Not penalised although not recommended: Use of “mobile charged particles” instead of “ mobile ions”. [2] Total [5]

A3 Concentration of H+ ions for the solution = 0.001 mol dm-3 Maximum concentration of H+ ions from arsenic acid = 0.0005 × 3 = 0.0015 mol dm3 [1] Since the concentration of H+ ions is lower than the maximum H+ ion concentration [1], it shows that arsenic acid does not dissociate fully [1] and is a weak acid. Accept: 0.001/0.0005=2 [1] At pH 3, 2 moles of H+ ions are produced per mole of acid but H3AsO4 should be tribasic. [1] This shows that arsenic acid does not dissociate fully [1] and is a weak acid. [3] Total [3] A4 (a)(i) Gas syringe / eudiometer Accept: syringe [1] (a)(ii) C12H22O11 [1] (a)(iii) Assuming 100 g of compound, C H O mass / g 41.5 6.40 52.1 molar mass / g mol-1 12.0 1.0 16.0 no of moles [1] 3.458 6.40 3.256 simplest ratio 1.062 1.965 1 mole ratio 1 2 1 empirical formula = CH2O [1] [2] (a)(iv) When the number of atoms of some elements are very close together, there is a possibility that they appear to be the same number. Accept: The empirical formula method involves rounding off values to the nearest whole number, resulting in inaccuracies. Reject: It is inaccurate because the ratios are not the same. [1] (b)(i) Filtration [1] (b)(ii) Crystallisation [1] Total [7] A5 (a) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3 [1] (b)(i) The two isotopes have different numbers of neutrons / numbers of nucleons / nucleon mass / atomic mass. [1] (b)(ii) Let x be the percentage abundance of 73As 74.9 = [75(100 – x) + 73x] ÷ 100 [1] 2x = 10 x =5.00 % [1] [2]

(c) As arsenic molecules sublime, (arrangement) the molecules that were initially closely packed will move far apart [1]; (movement) molecules that were vibrating about fixed positions will move at high speeds [1] as a gas; (exp