# The Ksp for Fe(OH)_3 = 2.5 x 10^-20. How would you find the Ksp equation in terms of Fe^+3 and OH^- concentrations?

Aug 22, 2016

${K}_{s p} = \left[F {e}^{3 +}\right] {\left[H {O}^{-}\right]}^{3}$

#### Explanation:

We need (i) a chemical equation:

$F e {\left(O H\right)}_{3} \left(s\right) r i g h t \le f t h a r p \infty n s F {e}^{3 +} + 3 H {O}^{-}$

And (ii) a ${K}_{s p}$ expression:

${K}_{s p} = \left[F {e}^{3 +}\right] {\left[H {O}^{-}\right]}^{3}$

Given the normal conventions, i.e. if $S$ represents solubility etc., then,

${K}_{s p} = \left[S\right] {\left[3 S\right]}^{3}$ $=$ $27 {S}^{4}$ $=$ $2.5 \times {10}^{-} 20$.

S=""^4sqrt{(2.5xx10^-20)/(27)}

Of course hydroxide ion or iron concentration may already be artificially high.