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 both `FeSO_(4)` and `Na_(2)S` solutions before mixing =x mol`L^(-1)` =xM
As the equal volumes of the solution have been mixed , the concentrations of the solutions as well as of ions on mixing will be reduced to half `i.e., x//2M`
For FeS, the solubility product `(K_(sp)) = 6.3 xx 10^(-18)`(given)
`FeS(s) hArr underset(x//2)(Fe^(2+)) (aq) +S^(2-) underset(x//2)((aq))`
`((x)/(2)) xx ((x)/(2)) =6.3 xx 10^(-18)`
` "or "" " x^(2) =4 xx 6.3 xx10^(-18) =25.2 xx10^(-18)`
`x=(25.2 xx 10^(-18))^(1//2) =5.02 xx 10^(-9)M`
The maximum concentration of both the solutions is `5.02 xx 10^(-9) M`