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 water of crystallisation must be included in the calculation of M r of hydrated copper(II) sulfate. • It was necessary to describe how to weigh out the calculated mass, being specific as to how to subtract the mass of the weighing bottle. • It was expected to describe that the empty bottle should be weighed and how to use the result. • There was a need to recognise appropriate apparatus to be used to make up the standard solution. • Sufficient practical details to the steps in making up a standard solution were required. Preparation of suitable range of diluted solutions (from standard solution) of accurate concentrations (between 0 and 2.00 mol dm–3) Procedure: 1. Using a 50 cm 3 burette, transfer 50 cm 3 of standard solution into a 100 cm3 conical flask. This is ‘Reference Solution 1’. 2. Using the same 50 cm 3 burette, transfer 75 cm3 of standard solution into a 100 cm3 volumetric flask. 3. Top up the volumetric flask to the mark with deionised water carefully. 4. Stopper and s hake the flask well to obtain a homogenous solution of CuSO4(aq) (of concentration 1.50 mol dm–3). [Concentration of diluted solution = (75)(2.00)/100 = 1.50 mol dm–3] 5. Repeat steps 2 to 4 using different volumes of the standard solution as shown in Table 2. Reference Solution Volume of 2.00 mol dm–3 standard solution used / cm3 [CuSO4] in Reference Solution / mol dm–3 1 – 2.00 2 75.00 1.50 3 50.00 1.00 4 25.00 0.50 5 12.50 0.25 Table 2 [2]

2020 ASRJC/CHEM 3 Preparation of [Cu(NH3)4(H2O)2]2+ solutions (Mole ratio of Cu2+ : NH3 = 1 : 4) Procedure: 1. Pipette 10.0 cm3 of ‘Reference Solution 1’ (2.00 mol dm–3) into a 100 cm3 conical flask. 2. Using a 50 cm3 burette, transfer 50 cm3 of 2.0 mol dm–3 aqueous ammonia into the same conical flask. 3. Swirl the conical flask to ensure complete reaction to obtain ‘Complex S olution 1 ’ of concentration 0.333 mol dm–3. [Concentration of ‘Complex Solution 1’ = (10)(2.00)/(10 + 50) = 0.333 mol dm–3] 4. Repeat steps 1 to 3 using ‘Reference Solutions 2 to 5’ and Solution X, keeping the total volume constant (i.e. 60 cm3) for all complex solutions, to obtain ‘Complex Solutions 2 to 5’ and ‘Unknown’. [2] Comments: • It was necessary to describe a workable method to prepare at least five reference solutions of different concentrations from the 250.0 cm3 standard solution prepared. • The c oncentrations of the reference solutions should be well spread between 0 to 2.00 mol dm–3. • Any suitable combination of pipettes, burettes and/or volumetric flasks should be used. • Less precise apparatus such as measuring cylinders were not acceptable. • There was a n eed to ensure sufficient 2.0 mol dm –3 ammonia solution was added to each reference solution to form the [Cu(NH3)4]2+ complex ion, while also keeping the total volume constant for all their solutions. Many students did n ot add sufficient ammonia or omitted carrying out this part with the unknown solution X, as well as the reference solutions. Obtaining calibration line and determination of concentration of copper(II) ions in solution X Procedure: 1. Using a dropper, place a few cm3 of ‘Complex Solution 1’ inside the spectrometer. 2. Set the spectrometer to use the wavelength of orange light. 3. Obtain the absorbance value of ‘Complex Solution 1’. 4. Repeat steps 1 to 3 for ‘Complex Solutions 2 to 5’ and ‘Unknown’. 5. Plot a graph of absorbance against concentration of ‘Complex Solutions 1 to 5’ to obtain the calibration line. 6. Using the recorded absorbance value A, obtain the [complex]Unknown from the graph. Complex Solution [CuSO4] in Reference Solution / mol dm–3 Vol. of Reference Solution used / cm3 Vol. of NH3(aq) used / cm3 Conc. of Complex Solution / mol dm–3 Absorbance 1 2.00 10.0 50.00 0.333 2 1.50 10.0 50.00 0.250 3 1.00 10.0 50.00 0.167 4 0.50 10.0 50.00 0.0833 5 0.25 10.0 50.00 0.0417 Unknown [Cu2+]solution X 10.0 50.00 [complex]Unknown A [1]

2020 ASRJC/CHEM 4 Alternative Procedure: 1. Using a 50 cm3 burette, transfer 10.0 cm3 of ‘Standard Solution’ (2.00 mol dm–3) into a 100 cm3 conical flask. 2. Using a 50 cm3 burette, transfer 50 cm3 of 2.0 mol dm–3 aqueous ammonia into the same conical flask. 3. Swirl the conical flask to ensure complete reaction to obtain ‘Complex Solution 1 ’ of concentration 0.333 mol dm–3. [Concentration of ‘Complex Solution 1’ = (10)(2.00)/(10 + 50) = 0.333 mol dm–3] 4. Repeat steps 1 to 3 using a 50 cm 3 burette to make up, with deionised water, the total volume in the same conical flask consta