PJC_H2_CHEM_P2 ANS

Pioneer Junior College H2 Chemistry Preliminary Examination 2009 Paper 2 Answer Scheme 1(a)(i) Relative atomic mass of an element is defined as the ratio of the average mass of one atom of the element to 121 the mass of an atom of 12C. (ii) Let the percentage abundance of Ga-69 be x. 69x + (1-x)71 = 69.7 x = 0.65 [1] Therefore percentage abundance of Ga-69 be 65% ; percentage abundance of Ga-71 be 35%. (iii) A lCl 3 – simple covalent/molecular structure – weak temporary induced dipole-induce dipole attraction between molecules. PC l5 – simple covalent/molecular structure – permanent dipole - permanent dipole attractions between molecules. A lCl 3 exist as dimers, with stronger id-id (due to greater no. of electrons) than pd-pd of PCl5. Hence m.p. of A lCl3 is higher than PCl5. (b) (i) pH (ii) A l2O3 insoluble in water ; high lattice energy(pH 7) P 4O10 + 6H2O → 4H3PO4 (pH about 2-3) (c)(i) MgCO 3 will decompose first because it is thermally less stable: ƒ Mg2+ has smaller ionic radius than Ca2+ ƒ Mg2+ (higher charge density than Ca2+) is more polarising than Ca2+. ƒ Mg2+ polarises C-O bond of CO 3 2- more easily/to a greater extent, weakens and breaks the C-O bon at a lower temperature Na2O MgO Al2O3 SiO2 P4O10 SO3 7 -

(ii) M(OH) 2.nH2O → M(OH)2 + n H2O M(OH) 2 → MO + H2O Mass of anhydrous M(OH) 2 = 1.575 – 0.720 = 0.855 g Mass of water lost = 0.090 g Amount of water lost from anhydrous M(OH) 2 = 5.00 x 10-3 No. of moles of M(OH) 2.n H2O = 5.00 x 10-3 No. of moles of water from M(OH) 2.nH2O = 0.720/18.0 = 0.040 5.00 x 10 -3 mol of M(OH)2.nH2O ≡ 0.040 mol of H2O 1 mol of M(OH) 2. n H2O ≡ 8 mol of H2O n = 8 M r of M(OH)2 = = 171 A r of M = 137 M is barium 2(a) (i) CH 4(g) + H2O (g) CO(g) + 3H 2(g) Initial/atm 1 1 0 0 Δ/atm - 0.43 -0.43 +0.43 +(0.43) x 3 Equilibrium/atm 0.57 0.57 0.43 1.29 Total Pressure = 0.57 + 0.57 + 0.43 + 1.29 = 2.86 atm (ii) Kp = ) p )( p ( ) p )( p ( O HCH 3 H CO 24 2 [1m] K p = = 2.84atm2 (iii) 1. By Le Chatelier’s Principle, the position of equilibrium will shift to the right to remove some of the added heat. The forward endothermic reaction is favoured; as a resu