The average pH of open ocean water is 8.1. What is the maximum value of [Fe3+] in pH 8.1 seawater if the Ksp of Fe(OH)3 is 1.1*10^-36?

Question

chemistry - DrBob222, Friday, December 7, 2012 at 10:14pm
Fe(OH)3 ==> Fe^3+ + 3OH^-
pH = 8.1
pH + pOH = pKw = 14
Therefore, pOH = 14-8.1 = ?
pOH = -log(OH^-) and
(OH^-) = ?
Ksp = (Fe^3+)(OH^-)^3
Substitute Ksp and OH^- from above into Ksp expression and solve for Fe^3+.

I cant seem to get the right answer. I have no idea if I'm doing it right. So when you subtract 8.1 from 14, you get 5.9. Is that the value for OH that you plug into the pOH equation?

Answer 1

No, it isn't OH^-. That's the pOH. Then
pOH = -log(OH^-) and calculate OH from that, then substitute into Ksp expression and solve for Fe&3+