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)=6.3xx10^(-18))`.

Explanation

Let the concentration of each of `FeSO_4` and `Na_2S` is x mol `L^(-1)`. Then after mixing equal volumes :
`[FeSO_4]=x/2 M, [Na_2S]=x/2` M
or `[Fe^(2+)]=x/2 M` and `[S^(2-)]=x/2` M
`FeS hArr Fe^(2+) + S^(2-)`
`K_(sp)=[Fe^(2+)][S^(2-)]`
`6.3xx10^(-18)=x/2 xx x/2`
or `x^2=4xx6.3xx10^(-18)=25.2xx10^(-18)`
or `x=5.02xx10^(-9) "mol L"^(-1)`
Maximum concentration of `FeSO_4` and `Na_2S` which will not precipitate iron sulphide =`5.02xx10^(-9) "mol L"^(-1)`