pH of a saturated solution of (CaOH)₂ is 9. The solubility product (K_sp)  of Ca(OH)₂ is 

pH of the saturated solution of Ca(OH)₂=9 
$\therefore$ pOH of the saturated solution of Ca(OH)₂ =5 
$\begin{array}{l}\Rightarrow \left[{\mathrm{OH}}^{-1}\right]={10}^{-5}(\because \mathrm{pH}+\mathrm{pOH}=14)\text{. }\\ 1/2\times {10}^{-5}{10}^{-5}\\ {\mathrm{K}}_{\text{sp }}=\left[{\mathrm{Ca}}^{2+}\right]{\left[{\mathrm{OH}}^{-}\right]}^2=\left[1/2\times {10}^{-5}\right]{\left[{10}^{-5}\right]}^2=0.5\times {10}^{-15}\end{array}$