Precipitation when $Q_{s p}=K_{s p}$ $$ \begin{aligned} & {\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^2=10^{-11}} \\ & 0.1 \times\left[\mathrm{OH}^{-}\right]^2=10^{-11} \Rightarrow\left[\mathrm{OH}^{-}\right]=10^{-5} \\ & \Rightarrow \mathrm{pOH}=5 \Rightarrow \mathrm{pH}=9 \end{aligned} $$