chemistry solutions

Question

A chemist needs to create a series of standard Cu2 (aq) solutions for an absorbance experiment. For the first standard, he uses a pipet to transfer 5.00 mL of a 2.93 M Cu2 (aq) stock solution to a 500.0 mL volumetric flask and adds enough water to dilute to the mark. He then uses a second pipet to transfer 10.00 mL of the second solution to a 100.0 mL volumetric flask and adds enough water to dilute to the mark. Calculate the concentration of the Cu2 (aq) solution in the 100.0 mL volumetric flask.

Answer 1

I thought I'd add an additional method to solve this problem. In a case like this, where the dilutions are simple multiples such as 5 ml diluted to 500 ml (1:100) and 10 ml diluted to 100 ml (1:10), this may be an easier method.

1:100 dilution x 1:10 dilution = 1:1000 dilution (overall).

2.93 M/1000 = 0.00293 M = 2.93x10^-3 M

Answer 2

 5.00 mL x 2.93 mol/1000mL Cu2 = .01465 mol of Cu2

.01465 mol of Cu2 in 500mL

.01465 mol of Cu2/.5L = .0293M Cu2

10.0 mL x .0293mol Cu2/1000mL = 2.93 x 10^-4 mol Cu2

2.93 x 10^-4 mol Cu2 in 100 mL

2.93 x 10^-4 mol Cu2/.1 L = 2.93 x 10^-3 M Cu2