The solubility products of Fe(OH)2 and Fe(OH)3 are 10^-17 and 10-38 respectively. If the concentrations of Fe2+ and Fe3+ are each 10^-5, at what pH will each hydroxide just begin to precipitate?

Answer 1

Here is how you do the Fe(OH)2. Fe(OH)3 is done the same way.

.......................Fe(OH)2 ==> Fe^2+ + 2OH^-
I.....................solid................0............0
C...................solid.................x...........2x
E...................solid.................x...........2x

Also, the problem tells you that x = (Fe^2+) = 1E-5. Substitute that into the Ksp expression and solve for OH^-, then convert that to pH.

Ksp = 1E-17 = (Fe^2+)(OH^-)^2

Post your work if you get stuck.