Ksp Problem??????????????????

Question

Magnesium fluoride dissolves in water to give a solution containing 8.0 x 10^-2 g solute per liter at 25 degrees C. Calculate Ksp for magnesium fluoride at this temperature.

Answer 1

MgF2

8.0x10^-2 g per L / 62.32 g/mol = 1.28x10^-3 mol per L

Ksp = [Mg++][F-]^2

[Mg+] = 1.2837x10^-3 M

[F-] = 2.5674x10^-3

Ksp = 8.46x10^-9

Answer 2

Compound is MgF2.  Mole wt is about 42.  So moles/L of salt added is 8x10-2/ 42 or about 1.9x10-3 M.  Since each molecule provides 2 ions of F, and 1 of Mg

Ksp = [1.9x10-3][3.8x10-3]^2 which is about 2.6x10-8