Question

The solubility product of Mg(OH)2 is 1.2 x 10^-11. What minimum OH- concentration must be attained (for example, by adding NaOH) to decrease the Mg concentration in a solution of Mg(NO3)2 to less than 1.0 x 10^-10 M (molar)?

Answer 1

Mg(OH)2 ==> Mg^+2 + 2OH^-

Ksp = (Mg^+2)(OH^-)^2 = 1.2 x 10^-11

You know (Mg^+) and Ksp, solve for (OH^-).
It's that simple.

Answer 2

0.109