Question

What is the solubility (in mol/L) of iron (III) in a solution of pH=8.1? The Ksp for this salt is 1.0E-36.

Answer 1

`sf(4.4xx10^(-10)color(white)(x)"mol/l")`

Explanation:

You have not quoted the name of the salt but at this high pH iron(III) ions will precipitate out as iron(III) hydroxide:

`sf(Fe_((aq))^(3+)+3OH_((aq))^(-)rarrFe(OH)_(3(s)))`

I will assume that the `sf(K_(sp))` value refers to this.

So we have:

`sf(Fe(OH)_(3)rightleftharpoonsFe^(3+)+3OH^(-))`

`sf(K_(sp)=[Fe^(3+)][OH^-]^(3)=1.0xx10^(-36))`

`sf([OH^(-)]=3xx[Fe^(3+)])`

If we let `sf([Fe^(3+)]=x)` then:

`sf(x.(3x)^3=1.0xx10^(-36))`

`sf(x.27x^(3)=1.0xx10^(-36))`

`sf(27x^(4)=1.0xx10^(-36))`

`sf(x^(4)=1/(27)xx1.0xx10^(-36)=0.03704xx10^(-36))`

`:.` `sf(x=""^4sqrt(0.03704xx10^(-36))color(white)(x)"mol/l")`

`sf(x=4.39xx10^(-10)color(white)(x)"mol/l")`

This will be the solubility of iron(III) at that particular temperature.