What is the maximum concentration of equimolar solutions of ferrous sulphate and sodium sulphide so that when mixed in equal volumes, there is no precipitation of iron sulphide? (For iron sulphide, `K_(sp) = 6.3 × 10^(-18)`).

Explanation

Suppose the concentration of each of `FeSO_(4)` and `Na_(2)S` is `x "mol" L^(-2)`. Then after mixing equal volumes,
`[FeSO_(4)]=[Na_(2)S]=x/2 M`, i.e., `[F^(2+)]=[S^(2-)]=x/2 M`
`K_(sp)` for `FeS=[Fe^(2+)][S^(2-)]`, i.e., `6.3xx10^(-18)=x/2xx x/2`
or `x^(2)=25.2xx10^(-18)`, or `x=5.02xx10^(-19) M`
Hence the highest molarity for the solution is `5.02xx10^(-9)M`.