Question

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, $K_{sp}=6.3\times {10}^{-18}$).

Answer 1

After Mixing,

Let $\left[FeS{O}_4\right]=\left[N{a}_{2}S\right]=\frac{x}{2}M$

$\left[F{e}^{2+}\right]=\left[FeS{O}_4\right]=\frac{x}{2}M$

Also, $\left[S^{2-}\right]=\left[N{a}_{2}S\right]=\frac{x}{2}M$

$FeS\leftrightarrow F{e}^{2+}+S^{2-}$

$Ksp=\left[F{e}^{2+}\right]\left[S^{2-}\right]$

$6.3\times {10}^{-18}=\left(\frac{x}{2}\right)\left(\frac{x}{2}\right)$

$x=5.02\times {10}^{-9}$

Maximum Concentration of both solutions should be $5.02\times {10}^{-9}M$ for no precipitation.