Equilibrium problem, initiating a precipitate..

Question

Calculate the minimum concentration of Mg2 that must be added to 0.10M NaF in order to initiate a precipitate of magnesium flouride. (For MgF2, Ksp = 6.9 x 10^-9)

Can someone please show me step by step how to solve this problem? The answer is 6.9 x 10^-7.

Answer 1

Definition: when precipitation just begins, [Mg2+][F-]^2 = Ksp

(Reasoning: Ksp is just the equilibrium constant for the solid going into solution)

You know Ksp, and you are told what [F-] is in this case, so all that remains is the arithmetic.