Question

The absorbances of a series of solutions containing Cr3+ were measured at a fixed wavelength and a path length of 1.1 cm. The following data were recorded: 4.0 mL of 0.100 M Cr3+ solution; 1.0 mL of water; Absorbance = 0.807; Calculate the concentration of Cr3+.

Answer 1

you said "a series of solutions" how come there is only one data point?

And anyway, just use the Beer-Lambert law, A = ebc, with a is the absorbance which is 0.807, e is a constant called molar absorptivity (or extinction coefficient, they're really the same thing), b is the path length, which is 1.1 in this case.

You need to calculate e if they don't give you the value for it. In case you have to calculate it, you need more than one point of data. Just plot those data on an xy graph, then find the slope, that's your e.

Now you have everything needed to solve for c, the concentration :)