when Ksp is the solubility product constant for a solid substance when it dissolved in the solution. and measure how a solute dissolves in the solution.
and when the solubility is the maximum quantity of solute which can dissolve in a certain solution.
since the reaction equation is:
Mg(OH)2 → Mg2+ + 2OH-
∴ Ksp expression = [Mg2+][OH]^2
when PH = 9.47
and PH+ POH = 14
∴ POH = 4.53
when POH = -㏒[OH-]
∴[OH-] = 2.95 x 10^-5
by substitution on Ksp expression:
8.9 x 10^-12 = [Mg2+] (2.95 x 10^-5)^2
∴ [Mg2+] = 0.01 M
∴ the molar solubility = 0.01 M