HCI_H2_CHEM_P2_ANSWERS Prelim

2014 HCI C2 Chemistry Prelim Paper 2 – Answers 1 (a) Cathode: 2H+(aq) + 2e-  H2(g) [1] (b) Q = It = (0.50)(10)(60) = 300 C No. of moles of electrons = 300 / 96500 = 0.003109 No. of moles of hydrogen gas = 1.554 x 10-3 Volume of hydrogen gas = 1.554 x 10-3 x 24000 = 37.3 cm3 [1] (c) Diagram – 3 marks:  Battery (correct polarity)  complete circuit with ammeter  appropriate choice of electrodes – Pt or graphite  appropriate choice of apparatus for collecting H2 and holding electrolyte – E.g. inverted measuring cylinder/burette, beaker/ water trough Electrolysis – 2 marks:  Record initial and final current to find average current  Stop experiment after 10 min  Record initial and final burette / measuring cylinder Titration – 3 marks:  Stir/swirl the electrolyte after disconnecting the set-up  Pipette out 25.0 cm3 of electrolyte into a conical flask, fill up burette  Record initial and final burette readings  Add starch indicator at correct juncture  State colour change – blue-black to colourless Calculations – 2 marks:  Determine number of moles of electrons at anode  Determine L using the equation L = (average current x time) / (number of moles of electrons x charge of an electron) Sample answer: 1) Using the given 500 cm3 of acidified KI, fill a 50 cm3 measuring cylinder with some acidified KI and transfer the remaining solution into the water trough. Invert the measuring cylinder over the Pt electrode and set up the apparatus as shown above. Do not connect the battery yet. 2) Record the initial reading on the measuring cylinder.

3) Connect the battery and immediately record the initial current shown on the ammeter. Start the stopwatch at the same time. 4) At 10 minutes, record the final current and final reading on the measuring cylinder. Disconnect the battery. Dismantle the setup and empty the solution in the measuring cylinder into the water trough. 5) Stir the electrolyte using a glass rod. 6) Pipette 25.0 cm3 of the electrolyte into a 250 cm3 conical flask. 7) Fill up a burette with aqueous Na2S2O3. Record initial burette reading. 8) Titrate the solution in the conical flask with aqueous Na2S2O3 until the solution turns pale yellow. Add about 1 cm3 of starch. 9) Continue titration until the solution turns from blue-black to colourless. Record final burette reading. Processing of Data Let the titre value be V cm3. No. of moles of Na2S2O3 used = = 1 x 10-5 V No. of moles of iodine used for titration = ½ x 1 x 10-5 V No. of moles of iodine produced in electrolysis = x ½ x 1 x 10-5 V No. of moles of electrons = 2 x x ½ x 1 x 10-5 V = 2 x 10-4 V Let average current be I Total charge passed through = It = I x (10 x 60) = 600I L = = - - Total: [12] 2 (a) (i) Mg2+(aq) + 2F–(aq)  MgF2(s) Ksp = [Mg2+][F-]2 mol3 dm-9 [1] (ii) [F−] = )19.0 101( 3  = 5.26 x 10-5 mo