What is the maximum concentration of equimolar solutions of `FeSO_(4) and Na_(2)S` so that when mixed in equal volumes, there is no precipitation of iron sulphide? (For iron sulphide, `K_(sp)=6.3xx10^(-18)`).

Explanation

`FeSO_(4)(aq)+Na_(2)S(aq)toFeS(s)darr+Na_(2)SO_(4)(aq)`
Let the molarity of `FeSO_(4)` and `Na_(2)S` solutions be xM. As these solutions are mixed in equal volumes, in the resulting solution `[Fe^(2+)]=(x)/(2)M and [S^(2-)]=(x)/(2)M`
There will be no precipitation of FeS if `[Fe^(2+)][S^(2-)] lt K_(sp)`
`therefore (x)/(2)xx(x)/(2) lt 6.3xx10^(-18) or , x lt 5.02xx10^(-9)M`
So, the maximum concentration of each of the solutions of ferrous sulphate and sodium sulphide is `5.02xx10^(-9)M`.