SAJC Prelim P1 P2 ANS

2009 H1 CHEMISTRY PRELIMS PAPER 1 MCQ Answer Key 1. B 6. B 11. C 16. B 21. C 26. D 2. C 7. D 12. C 17. A 22. C 27. C 3. A 8. C 13. D 18. B 23. B 28. B 4. B 9. C 14. D 19. D 24. A 29. A 5. A 10. A 15. B 20 B 25. C 30 C 2009 H1 CHEMISTRY PRELIMS PAPER 2 MARKS SCHEME SECTION A: 1. (a) (i) It is the ratio of the average mass of 1 atom of the element to 12 1 the mass of an atom of carbon-12. [1] (ii) From the data booklet, Ar of Ga = 69.7 69.7 69 0.64 (1 0.64) 0.36 69.7 44.16 70.9 x x x       x = 71 Relative isotopic mass of the other Ga isotope = 71 [½] [½] (b) (i) 5 C 2O4 2- + 2MnO4 - + 16H+ → 10CO2(g) + 2Mn2+ + 14H2O [1] (ii) No. of moles of MnO4 - used = 25.60 0.021000  = 5.12 × 10-4 mol No of moles of C2O4 2- in 25.0 cm3 = 4 55.12 10 2   = 1.28 × 10-3 mol No of moles of C2O4 2- in 250 cm3 = no. of moles of H2C2O4 =0.0128 mol Mass of H 2C2O4 in 250 cm3 = 0.0128 × 90 = 1.152 g Percentage purity = 1.152 1001.27  =90.7% [½] [½] [½] [½] 2. (a) Element W belongs to Group VI. There is a large increase between 6 th and 7th ionisation energy, suggesting that the 7th electron is removed from an inner shell. [1] [1] (b) (i) WO2 or WO3 [½]

Covalent bonding [½] (ii) pH = 2 [1] 3. (a) C1: sp2 C2: sp3 (b) Carbon Shape Bond angle C1 Trigonal planar 120° [1] C2 tetrahedral 109° [1] (c) [1] 4. (a) (i) Standard enthalpy change of formation of 2-methylpropene is the enthalpy change when 1 mole of 2-methylpropene is formed from carbon and hydrogen elements under standard conditions. [1] (ii) 4 C (s) + 4 H2 (g) → (CH3)2CH=CH2 (g) [1] (b) ∆Hf° (2-methylpropene) = 4 × ∆Hc° (C) + 4 × ∆Hc° (H2) - ∆Hc° (2-methylpropene) = 4(-393) + 4(-286) – (-2520) = - 196 kJ mol-1 [1/2] for ans [1/2] for units [1] [1] 5. (a) Show 2 half-lives on graph half-life = 10 s half-lives are constant hence 1 st order [½] [½] [1] (b) (i) : Max vol = 60 cm3 ; half-life = 20s (ii): Same half life as original curve = 10 s; [½] [½] [½] [½]

max vol = 144 cm3 6. (a) KC =    Y X Z 2 =   4 . 005 . 0 08 . 0 04 . 02  [1/2] for correct substitution; [1/2] for ans KC = 0.4 [1] [1] (b) 0.05 mol dm -3 of Y was added into the system. [1] (c) show all 3 concentrations halved no change in equilibrium position [½] [½] 7. (a) A buffer solution is a solution whose pH does not change significantly when a small amount of acid or base is added to it. [2] 0.10