DHS Prelim P2_Answer_scheme

1  DHS 2015 9647/02 2015 DHS Year 6 H2 Chemistry Preliminary Examination (Answer Scheme) 1 Planning (P) 1 (a) (i) 1. Connect the nozzle of hydrogen gas can to the effusion hole of a 100 cm 3 well- greased syringe using rubber tubing. 2. Flush the syringe with hydrogen at least once before filling the syringe with hydrogen gas at constant temperature of 25 C. Piston should be above the start volume. 3. Detach the tubing from the effusion hole and allow hydrogen gas to effuse out. (Free movement, not pushed to start mark.) 4. Start the stopwatch when the piston passes the 80 cm 3 mark on the syringe and stop the stopwatch when the piston falls to 30 cm3 mark on the syringe. Start the stopwatch when the piston passes the 80 cm 3 mark on the syringe and stop the stopwatch after 10 minutes. 5. Repeat step 1 – 4 with another sample of hydrogen gas and obtain the average effusion time OR volume at constant temperature of 25 C. 6. Repeat step 1 – 4 with the four other gases (butane, oxygen, carbon dioxide and chlorine gas) at constant temperature of 25 C. Gas Mr Effusion time for expt 1 / s Effusion time for expt 2 / s (Average) effusion time / s Rate of effusion / cm3 s–1 Hydrogen 2.0 Butane 58.0 Oxygen 32.0 Carbon dioxide 44.0 Chlorine 71.0 OR Gas Mr Effusion volume in 10 min for expt 1/ cm3 Effusion volume in 10 min for expt 2 / cm3 Average effusion volume in 10 min/ cm3 Rate of effusion / cm3 min–1 Hydrogen 2.0 Butane 58.0 Oxygen 32.0 Carbon dioxide 44.0

2  DHS 2015 9647/02 (ii) Rate of effusion/ cm s–1 Mr 28.0 Using the results, plot a graph of rate of effusion against Mr of the gas, read off the rate of effusion of nitrogen gas, x, at its Mr of 28.0. (b) (i) Moles of Cu(NH3)4SO4 to be formed  Moles of CuSO4 required = 0.02196 mol Moles of NH 3 required  4  Moles of CuSO4 required = 0.08787 mol  Volume of CuSO4 required = 0.02196 / 0.50 = 43.9 cm3  Volume of NH3 required = 0.08787/ 2.0 = 43.9 cm3 (ii) Procedure 1. Using a 50 cm 3 measuring cylinder, transfer 50 cm3 of CuSO4 (aq) into a 150 cm3 beaker. 2. Using another 50 cm 3 measuring cylinder, gradually add 50 cm 3 of NH3 (aq) into the 50.00 cm3 of CuSO4 (aq), with stirring until all the solids have dissolved. 3. Leave the mixture to stand for 15 minutes undisturbed, so that an equilibrium can be established. 4. Place the beaker in an ice bath and slowly add (drop wise), using a dropper, about 30 cm3 of acetone to the beaker with continuous stirring. 5. Leave the solution to remain in the ice bath for about 15 minutes. 6. Filter off the crude product, using (vacuum) filtration (or Buchner apparatus), and wash it with a little cold aceton