Mg(OH)2 --> Mg+2 + 2OH-
Mg+2 = x
2OH- = 2x
Find a Ksp expression:
Ksp = [Mg+2][OH-]^2
1.2x10^-11 = [Mg+2][OH-]^2
Now plug in x for each of the concentrations in the Ksp equation, giving:
1.2x10^-11 = [x][2x]^2 simplify this equation
1.2x10^-11 = 4x^3 and solve for x,
x = 0.000144
because for every one Mg ion that is soluble, 2 OH ions are soluble, to solve for the concentration of OH (which is required to find the pH), you must multiply x by 2
[OH] = 2x, [OH] = 2(0.000144) = 0.000288
pOH = -log[OH]
pOH = -log(0.000288) = 3.54
pH = 14.00 - 3.54
pH = 10.4
