AJC 2013_ALevel Chem P2 solns (2020 version)

2020 ASRJC/CHEM 1 H2 Chemistry 9647 N2013 P2 Solutions 1 (a) [Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4(H2O)2]2+ + 4H2O or [Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4]2+ + 6H2O [1] (b) Orange [1] [Cu(NH3)4]2+ is deep–blue in colour. Hence, its complementary colour (orange) will be most strongly absorbed. Comments: A significant number of candidates were not able to apply their understanding of colour in transition metal compounds to predict and explain that the most strongly absorbed colour would be the complementary colour to the blue colour of the complex. (c) General comments: Candidates were expected to describe how to use the correct apparatus for a given step, rather than just outlining generally what had to be done. Preparation of 250.0 cm3 standard solution of 2.00 mol dm–3 aqueous copper(II) sulfate No. of moles of CuSO4(aq) in 250 cm3 = 2.00 x 250/1000 = 0.500 mol Mass of CuSO4.5H2O to measured = 0.500 x [63.5 + 32.1 + 4(16.0) + 5(18.0)] = 0.500 x 249.6 = 124.8 g Procedure: 1. Weigh accurately 124.8 g of solid CuSO4.5H2O using a weighing bottle. Record the total mass of solid CuSO4.5H2O and weighing bottle. 2. Transfer the solid into a 100 cm3 beaker and add 50 cm3 deionised water to dissolve the solid completely. Add more deionised water to dissolve if necessary. 3. Transfer this solution carefully into a 250 cm3 volumetric flask using a glass rod and filter funnel. Rinse the beaker with deionis ed water a few times, and transfer the washings into the volumetric flask. 4. Top up the volumetric flask to the mark with deioni sed water. Add deionised water drop–wise to ensure the water level does not exceed the mark. 5. Stopper and shake the volumetric flask well to obtain a homogenous (standard) solution. 6. Re–weigh the empty weighing bottle. Record the readings in Table 1. Mass of CuSO4.5H2O and weighing bottle /g x Mass of residue and weighing bottle /g y Mass of CuSO4.5H2O used /g x – y = 124.800 Table 1 Concentration of standard solution = (124.800/249.6) / 0.250 = 2.00 mol dm–3 [1] [2]

2020 ASRJC/CHEM 2 Comments: • It was necessary to calculate the mass of solid hydrated copper(II) sulfate needed to make up 250.0 cm3 of 2.00 mol dm–3 solution of copper(II) sulfate. • The wate