The ion precipitating first when a solution containing $0.01 \mathrm{M}$ each of ${\mathrm{Pb}}^{2+},{\mathrm{Zn}}^{2+},{\mathrm{Mn}}^{2+}$ and ${\mathrm{Co}}^{2+}$ is saturated with ${\mathrm{H}}_2\mathrm{S}$  is (which provides $1.0\times {10}^{-14}\mathrm{M}$ of ${\mathrm{S}}^{2-}$n the solution) is 

  (Given: $K_{\mathrm{sp}}\left(\mathrm{PbS}\right)=2.3\times {10}^{-27}{\mathrm{M}}^2,K_{\mathrm{sp}}\left(\mathrm{ZnS}\right)=1.0\times {10}^{-21}{\mathrm{M}}^2,K_{\mathrm{sp}}\left(\mathrm{MnS}\right)=7.9\times {10}^{-13}{\mathrm{M}}^2$)

 

Lesser the value of $K_{\mathrm{sp}}$, earlier the salt precipitates. Hence, ${\mathrm{Pb}}^{2+}$ is precipitated first.

When cations and anions in an aqueous solution interact, a precipitate, an insoluble ionic solid, is created. The common ionic solids' solubility principles can be used to determine whether or not such a reaction takes place.