Ca(OH)2 is a strong base that dissociates in water to give two OH- ions per formula unit. Thus, the concentration of OH- ions in the solution is 2 x (0.011 M ) = 0.022M .We can calculate the pH of the solution in two equivalent ways .
First,we could use equation Kw = [H3O+][OH-] to calculate [H+]
and then use the equation pH = -log [H+] to calculate the pH
[H+] = 1.0 x 10-14 / 0.022 = 4.6 x 10-13 M
pH = -log [H+] = -log (4.6 x 10-13) = 12.34