The solubility of magnesium fluoride, MgF2, in water is 0.016 g/L. What is the s

Question

The solubility of magnesium fluoride, MgF2, in water is 0.016 g/L. What is the solubility (in grams per liter) of magnesium fluoride in 0.013 M sodium fluoride, NaF?

Explanation / Answer

0.011M NaF + MgF2 is common ion effect MgF2 --> Mg2+ + 2F- 0.016g/L; 0.016gMgF2 / 62.3g/mole = 2.57x10^-4moles for 2.57x10^-4M MgF2 this gives us 5.14x10^-4M F- in solution Ksp MgF2 = [Mg2+][2F-]^2 Ksp MgF2 = 2.57x10^-4 x (2 x 2.57x10^-4)^2 = 6.78x10^-11 0.011M NaF = 0.011M F- MgF2.....Mg2+.....F- some.....0..........0.011 less.....+x............+2x less......x..........0.011+2x Ksp MgF2 = 6.79x10^-11 = [x][0.011]^2.....Ksp is already very low so, 2x here is negligible 6.79x10^-11 = 1.21x10^-4x x = 5.61x10^-7M = 3.5x10^-5g/L

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