Finding the concentration with a Ksp value

What is the [Mg+2] in .10 M NaF that is saturated with MgF2 at 25°C?

Ksp for MgF2 = 6.4 x 10^-9

Thanks!

This is a trick question. Usually, they ask for [Mg2+] in a solution of MgF2 that also happens to have 0.10 M [F-]

Ksp = [Mg2+][F-]^2 = 6.4 x 10^-9

Let [Mg2+] = x. Then [F-] = 0.10 + 2x. But x is so small that we can neglect it and say that [F-] = 0.10

(x)(0.10)^2 = 6.4 x 10^-9

x = 6.4 x 10^-7 = [Mg2+]

AX Ksp =X^2
AX2 ksp=4X^3
AX3 Ksp= 27x^3
A2X3 Ksp=108x^5

you have AX2 6.4E-9 =4x^3 X=solubility in mol/L

but you have a common ion so ksp= (x)(0.10 -2x)^2= 0.01x (disregard the 2x compared to the 0.10)
solve for x