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.

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Answer 1 · 2018-04-22T15:55:03

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

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.