[Ciubba]

Calculate the solubilities of Cd(OH)₂(s) and Fe(OH)₃(s) as a function of pH, then determine at what pH range the two hydroxides can be separated.

I can get that the solubility of Cd(OH)₂ is (7.2*10¹³)*[H3O⁺]² and that the solubility of Fe(OH)3 is (2.8*10³)*[H3O⁺]³, which agrees with the book's answer. However, I cannot figure out how the book determined that the pH range of separability is from 2 to 8-- could someone point me in the right direction?

[Borek]

It means one precipitates, the other doesn't. Actually exact values are a function of concentrations of both cations, so the question - as stated - is not possible to answer precisely, unless your book defined what it means by separation.