Chemistry Concentration + Absorbance?

a)  Looking at the left-hand graph, an absorbance of 0.375 appears to correspond to a concentration of about 0.07 M.

b)

(0.07 mol/L) x (0.050 L) x (154.9958 g CoSO4/mol) = 0.54 g CoSO4

c)

(0.54 g CoSO4) / (0.800 g total) = 0.675 = 67.5% CoSO4

100% - 67.5% = 32.5% ZnSO4

d)

Presumably because one would not expect a white salt's solution to absorb significantly at 480 nm.  Although it would have been much better if the student had run a blank consisting of pure ZnSO4 in aqueous solution at 480 nm to test this assumption.

e)

i) The water droplets will dilute the sample, so the measured concentration will be lower than it should be.

ii) The fingerprints will block some of the light (decrease the transmittance) causing the absorbance to be artificially high, making the measured concentration higher than it should be.

iii) The right-hand graph shows that the absorbance at 520 nm is higher than at 480 nm, so the measured absorbance -- and so the measured concentration -- will be higher than it should be.