According to the balanced equation for this reaction:
Mg(OH)2(s) ⇄ Mg+2(aq) + 2OH-(aq)
when Ksp = [Mg+2][OH]^2
when we have Ksp = 2.06x10^-13 & [Mg+2] = 0.021 M
so by substitution:
2.06x10^-13 = 0.021 * [OH]^2
[OH]^2 = (2.06x10^-13) / 0.021 = 9.8x10^-12
∴ [OH] = 3x10^-6 M
So 3x10^-6 M is the minimum concentration of [OH -] required to precipitate Mg+2
