NYGH 2015-S3EOY-IP Chem P1 P2 Ans

1 2015 Sec 3 End-of-Year Exam Chemistry Answers Section A 1 B 2 B 3 D 4 B 5 B 6 D 7 B 8 D 9 B 10 A 11 C 12 A 13 C 14 C 15 A 16 D 17 B 18 B 19 D 20 C 21 B 22 C 23 A 24 B 25 A 26 D 27 B 28 D 29 B 30 D Section A No Answers Marks A1(a) The brown colour slowly spreads upwards until the mixture is homogeneous./Two jars will be uniformly brown/coloured. [1] A1(b) The rate of change increases. OR The change would be faster/Time taken for diffusion to occur will be shorter. [1] A1(c) If the bromine sample was impure, it would boil at a temperature higher than 58.8oC. Impure bromine would also boil over a range of temperatures. [1] [1] A2(a) precipitation [1] A2(b) Ca2+(aq) + CO3 2− (aq)  CaCO3(s) [1] A2(c) filtration [1] A3(a) Ionic [1]

2 No Answers Marks A3(b) Sodium nitride has a giant lattice/ ionic lattice/ crystal lattice structure. There are strong electrostatic forces of attraction between the sodium and nitride ions./oppositely charged ions. [1] [1] A3(c) Sodium nitride conducts electricity in the molten or aqueous state. OR Sodium nitride is soluble in water. Sodium nitride is brittle. [1] A3(d) monobasic, strong [2] A3(e) Na3N(s) + 4HCl(aq)  3NaCl(aq) + NH4Cl(aq) [2] A4(a) methyl orange or any other suitable indicator [3] A4(b) A strong acid is one which is completely ionized in water to form H+ ions. [1] A4(c) MgCO3(s) + H2SO4(aq)  MgSO4(aq) + H2O(l) +CO2(g) 2NaOH(aq) + H2SO4(aq)  Na2SO4(aq) + 2H2O(l) [1] [1] A4(d) Concentration of NaOH = 0.400 ÷ (23+16+1) = 0.0100 moldm−3 No. of moles of NaOH used in titration = 0.0100 × 20.50/1000 = 2.05 × 10−4 Mole ratio of NaOH to H2SO4 is 2:1 No. of moles of H2SO4 used in titration with NaOH = 2.05 × 10−4 ÷ 2 = 1.03 × 10−4 [1] [1] [1] A4(e) No. of moles of MgCO3 used = 0.400 ÷ (24+12+16×3) = 4.762 × 10−3 Mole ratio of MgCO3 to H2SO4 is 1:1 No. of moles of H 2SO4 used = 0.400 ÷ (24+12+16×3) = 4.76 × 10−3 [1] [1] A4(f) Total number of moles of H2SO4 = 2.05 × 10−4 ÷ 2 × 100/25.0 + 4.762 × 10−3 = 5.172 × 10−3 [1] [1]

3 No Answers Marks Concentration of H2SO4 = 5.172 × 10−3 ÷ 100/1000 = 0.0517 moldm−3 A4(g) Precipitation. Any aqueous magnesium salt plus any aqueous carbonate. [1] [1] A5(a) B: Ca(OH)2 C: AgCl Accept: chemical names [1] [1] A5(b) NH4 +(aq) + OH−(aq)  NH3(g) + H2O(l) [1] A5(c) NH4Cl and CaCl2 Accept: chemical names [2] A5(d) Calcium sulfate is insoluble in water. OR If the sulfate ion was present, A should be a white suspension / white solid in a colourless solution instead of just a colourless solution. [1] A6(a) concentration of H+ ions, surface area of zinc / particle size of zi