Prepare a ~ 0.020 M solution of Cu^2+ by accurately weighing the mass of CuSO_4
ID: 534692 • Letter: P
Question
Prepare a ~ 0.020 M solution of Cu^2+ by accurately weighing the mass of CuSO_4 middot 5H_2O you calculated in your Pre-lab Question. Dissolve in ~25 mL of DW and transfer quantitatively to a 100 mL volumetric flask. Make up to the mark with DW and mix well. This is your stock solution. Calculate the [Cu^2+] in this solution based on the mass just weighed to the correct number of significant figures. Empty some of this stock solution into a clean, dry 50 mL beaker and rinse a 10 mL graduated pipet. Using fresh stock solution, pipet known volumes of this stock Cu^2+ solution and known volumes of 0.5 M NH_3 into 15 cm test tubes as outlined Table 11.2. Mix each test tube well. Save these four solutions. Calculate the [Cu^2+] Which solvent is best for detecting small amounts of copper? Water or NH_3 (aq) (Circleone) Why? ____ Mass of CuSO_4 middot 5H_2O used to prepare stock solution 0.500g Show calculation of [Cu^2+] in stock solution. Show calculation of [Cu^2+] in solution 1 for the calibration curve.Explanation / Answer
5. Cu2+ = 0.01 M
6. NH3 forms a colored complex and hence is best for detecting small amounts of Cu2+ in solution
7. [Cu2+] in stock solution = 0.02 M
8. In diluted solutions [Cu2+]
Solution 1, [Cu2+] = 0.02 M x 1 ml/10 ml = 0.002 M
Solution 2, [Cu2+] = 0.02 M x 2 ml/10 ml = 0.004 M
Solution 3, [Cu2+] = 0.02 M x 3 ml/10 ml = 0.006 M
Solution 4, [Cu2+] = 0.02 M x 4 ml/10 ml = 0.008 M
From molar absorptivity of [Cu(NH3)4]2+ = 58 M-1.cm-1 [from literature
[Cu2+] in solution 1 = 0.064/58 = 0.0011 M
[Cu2+] in solution 2 = 0.243/58 = 0.0042 M
[Cu2+] in solution 3 = 0.303/58 = 0.005 M
[Cu2+] in solution 4 = 0.406/58 = 0.007 M