The solubility product of ${\mathrm{Hg}}_2{\mathrm{I}}_2$ is equal to

$$\begin{gathered} {\mathrm{Hg}}_2{\mathrm{I}}_2\rightleftharpoons {\mathrm{Hg}}_2^{2+}+2{\mathrm{I}}^{-} \\ {\mathrm{K}}_{\mathrm{sp}}=\left[{\mathrm{Hg}}_2^{2+}\right]{\left[{\mathrm{I}}^{-}\right]}^2 \end{gathered}$$mercurous ion is in dimeric form which exist as ${\mathrm{Hg}}_2^{+2}$ not simply as ${\mathrm{Hg}}^{+}$