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 required concentration is x M . On mixing equal volumes , concentration gets halved. Therefore
` " " [Fe^(2+) ] = (x)/(2) and [S^(2-)] =(x)/(2)`
` " "Fe^(2+) +S^(2- ) to FeS `
For the state of no precipitations.
` " " [Fe^(2+) ] [S^(2-)] = K_(sp)`
` or " " (x)/(2).(x)/(2) = 6.3 xx 10 ^(-18) `
` or " " x= (6.3 xx10^(-8) xx 4) ^(1//2) = 5.02 xx 10 ^(-9) " mol " L^(-1)`