Calculate the solubility (in moles per liter) of Fe(OH)3?

Question

Calculate the solubility (in moles per liter) of Fe(OH)3 (Ksp = 4 10-38) in each of the following.

(a) a solution buffered at pH = 3.8
(b) a solution buffered at pH = 12.8

Answer 1

(a) If pH=3.8, pOH=11.2. Then [OH-]= 10^-11.2
=10-12x10^.8 = 6x10-12 appx
To find solubility, [Fe+3][OH-]3=Ksp
Assuming that solution is suitably buffered, OH- will be at 6x10-12 mole/L.
[Fe+3](6x10-12)^3 = 2.2x10-34(Fe+3)=4x10-38. [Fe+3]= 1.8x10-4 Mol/L, which is the limit to ferric hydroxide solubility at this pH.

If you can follow the above, just do it again for pH=12.8 You will find [OH-] is 9 orders of magnitude higher, and that Fe+3 limit will be 27 orders of magnitude LOWER than at pH 3.8.