What pH is required to start precipitation of Fe(OH)3 from a 0.380 M solution of FeCl3? Ksp of Fe(OH)3 = 4.0e-29.

Question

What pH is required to start precipitation of Fe(OH)3 from a 0.380 M solution of FeCl3? Ksp  of Fe(OH)3 = 4.0e-29.

DrBob222 · Answer

Fe(OH)3(s) ==> Fe^+3 + 3OH^-
Ksp = (Fe^+3)(OH^-)^3 = 4.0E-29
(OH^-) = cube root(4E-29/Fe^+3)
(OH^-) = 4.68E-10
Convert that to H^+ and to pH. I get 4.67 for pH.
I have ignored any contribution to the acidity made by the hydrolysis of the FeCl3.