HCI Prelim ANSWERS P2

2015 HCI C2 H2 Chemistry Preliminary Exam / Paper 2 1 HCI 2015 C2 CHEMISTRY PRELIM PAPER 2 ANSWERS 1 (a) Ammonia is more soluble in water than it is in trichloromethane as the molecules are able to form hydrogen bonds with water molecules. Hence, [NH 3(aq)] is higher than [NH3(trichloromethane)]. [1] (b) M2+(aq) + nNH3(aq) ⇌ [M(NH3)n]2+(aq) [1] or [M(H2O)6]2+(aq) + nNH3(aq) ⇌ [M(NH3)n]2+(aq) + 6H2O(l) Addition of acid (to the aqueous layer) removes the ‘free’ ammonia. Position of equilibrium shifts to the left. The titration results will give the total amount of ammonia (‘free’ plus complexed) in the aqueous layer. [1] (c) 1) Using separate burettes, place 50.0 cm 3 of aqueous M 2+, 50.0 cm 3 of aqueous ammonia and 100.0 cm3 of trichloromethane into a stoppered bottle. 2) Stopper the bottle and shake vigorously for about 5 minutes before leaving to equilibrate to room conditions. 3) Use a dry pipette to transfer 25.0 cm3 of the bottom organic layer into a conical flask. (Place a finger over the top of pipette or fit the pipette filler before lowering into the bottom layer. This will minimise the amount of top aqueous layer that enters the pipette.) 4) Add an equal volume of water and 2 drops of methyl orange indicator. 5) Fill a burette with hydrochloric acid. Record the initial burette reading. 6) Titrate the ammonia with hydrochloric acid (shaking between each addition) until the indicator changes colour from yellow to orange. Record the final burette reading. 7) Repeat the titration until titres are within 0.10 cm3 of one another. M1  apparatus to measure M2+(aq), NH3(aq) and CHCl3 (burettes / pipettes)  for mixing without spillage (stoppered bottle / stoppered conical flask / separating funnel). M2  volume of M2+(aq) = 50 cm3, volume of NH3(aq) > 30 cm3 M3  volume of CHCl3 allows at least three titrations (e.g. > 325cm3 or 310cm3)  allows mixture to reach equilibrium (e.g. shake for 5 minutes / shake and allow to stand) M4  apparatus for titration (dry pipette, burette & conical flask)  suitable indicator (e.g. methyl orange) and correct colour change at end -point (e.g. yellow to orange). M5  uses dry pipette to remove bottom organic layer; for separating funnel, collects bottom layer into dry beaker or dry conical flask before withdrawing aliquot with a dry pipette. M6  adds equal volume of water to organic layer before titration M7  records initial and final burette readings  repeats titrations until titre values are 0.10 cm3 (d) (i) [NH3(aq)] = x / 0.4 = 2.5x mol dm–3 [1] (NH3) added = 1.0 mol Total ‘free’ ammonia = x + (2.5x)(2) = 6x mol (volume of aqueous layer is 2 dm3) (M2+) = 0.1 mol (NH3) complexed = (1 – 6x) mol n = h(NH3)complexed h(M2+) = 10 (1 – 6x) [1]

2015 HCI C2 H2 Chemistry Preliminary Exam / Paper 2 2 2 (a) (i) [H+] = 10–9.2 = 6.31  10–10 mol dm–3 [1] [S2–] = 1.0 ´10-23 (6.31´10-10)2 = 2.51  10–5 mol dm–3