\(K_{s p}=\left[M g^{++}\right]\left[O H^{-}\right]^{2}\)
\(1.0 \times 10^{-11}=10^{-3} \times\left[O H^{-}\right]^{2}\)
\(\left[O H^{-}\right]=\sqrt{\frac{10^{-11}}{10^{-3}}}=10^{-4}\)
\(\therefore p O H=4\)
\(\therefore p H+p O H=14\)
\(\therefore p H=10\)