Question

What is the maximum concentration of equimolar solutions of ferrous sulphate and sodium sulphide so that when mixed in equal volumes, these is not precipitation of iron sulphide?

(For $FeS:K_{sp}=6.3\times {10}^{-18}$)

Answer 1

$K_i=\left[F{e}^{2+}\right]\left[S^{2-}\right]=\left[\frac{n}{2V}\right]\left[\frac{n}{2V}\right]=\frac{n^2}{4V^2}=\frac{1}{4}[M{]}^2$

as Molarity, $M=\frac{n}{V}$

if $K_i>K_p$, precipitation takes place.

Thus, $K_i=K_{sp}$, no precipitation takes place at this condition,

$\Rightarrow \frac{1}{4}[M{]}^2=6.3\times {10}^{-18}$

$\Rightarrow \left[M\right]=5\times {10}^{-9}moles/L$

Thus, this is the maximum concentration of solution, upto which no precipitation takes place.