Question

What is the solubility of MgF2 in .10M HF solution that has been buffered to have a pH of 4.00? Ka of HF = 3.5x10^-4

a) .19M b) 6.5x10^-7M c) 2.2x10^-2M d) 8.9x10^-10 M e) 1.1x10^-6 M

Answer 1

answer : e) 1.1 x 10^-6 M

HF   --------------->   H+ +   F-

0.10                         0         0

0.10 - x                    x          x

Ka = x^2 / 0.10 - x

3.5x10^-4 = x^2 / 0.10 - x

x = 5.74 x 10^-3

Ksp = [Mg2+][F-]^2

5.16 x 10^-11 = S x 0.00574^2

S = 1.1 x 10^-6 M