Question

A 0.200 g sample containing Cu is dissolved, and a diethyldithiocarbamate
coloured complex is formed. The solution is then diluted to 50.0 mL and the
absorbance measured as 0.260. A 0.500 g sample containing 0.240% Cu is
treated in the same manner, and the resulting solution has an absorbance of
0.600. Calculate the percentage of Cu in the sample. What is the molar
absorptivity of the Cu complex formed?

Answer 1

The easy way to do this is to work in g and A units. You know 0.5g sample x 0.240E-2 = 1.2E-3 g Cu. You know the ratio of A unit is 0.250 vs 0.6; therefore, 1.2E-3 g Cu x (.260/0.6) = ? g Cu in the unknown.

To calculate molar absorptivity use
A = ebc. Technically I don't think you can calculate this since the cell length is not given. However, in most cases this is 1 cm so
A = 0.6
b = 1 and c is calculated as follows:
(0.5g x 0.24E-2 x (1 mol/63.55) = mols Cu and convert to M by mols/L (L is 0.050 from dissolving in 50 ml). Then substitute and solve for e.