Describe how you would prepare a 10 m L sample of a standard solution that is a
ID: 991820 • Letter: D
Question
Describe how you would prepare a 10 m L sample of a standard solution that is a 4/5 dilution of the stock solution. Calculate the concentration of Cu'* ton In this standard solution if the stock solution was 0.0200 M Cu^2+. Suppose you have a copper nitrate solid. You are not sure, however, if the solid is copper (I) nitrate. CuNO,. or copper (II) nitrate. Cu(NO,), You dissolve a 0.1015 gram sample of this solid in water and nitric add and then dilute it to a final volume of 100. ml You treat a sample of this solution with ammonia and then measure the absorbance of the resulting blue-violet solution. By comparing the absorbance of this solution to that of standard copper solutions you determine the concentration of Cu" to be 5.41 x 10 ' M. Use this information (and the hints below) to find the mole ratio of Cu: NO, and determine the chemical formula for the original copper nitrate sample.Explanation / Answer
1. To prepare a 4/5 (1/1.25) dilution from stock solution.
Total volume = 10 ml
dilution factor = final volume of solution/initial volume of stock
Initial volume of stock = 10 ml/1.25 = 8 ml
So we will take 8 ml of stock solution and dilute it with another 2 ml of solvent.
If the stock was [Cu2+] = 0.02 M
concentration of [Cu2+] in the final solution = 8 ml x 0.02 M/10 ml = 0.016 M
2. [Cu2+] found by experiment = 5.41 x 10^-3 M
1) moles of Cu2+ present = 5.41 x 10^-3 M x 0.1 L = 5.41 x 10^-4 mols
2) mass of Cu2+ present = 5.41 x 10^-4 mol x 63.546 g/mol = 0.0344 g
3) mass of NO3- present = 0.1015 - 0.0344 = 0.0671 g
4) moles of NO3- = 0.0671 g/62.0049 g/mol = 1.0822 x 10^-3 mols
5) ratio Cu : NO3 = 1 : 2