"Unbuffered solution" means that OH-
ions only come from the dissociation of Ca(OH)2
(those from the dissociation of water are negligible). Then Ksp
, where x is the concentration of calcium ions and [OH-
] = 2x.
In fact, there is one more equilibrium Ca(OH)2
in the calcium hydroxide solution, which is not taken into account by the solubility product. As a result, the concentration of calcium ions calculated from the actual pH value of the solution is slightly lower (a few%) than half of the concentration of hydroxide ions. With your Ksp
value, nonsense comes out of your incorrect calculation.