Given the following equilibrium systems:
CaF2(s) <----> Ca2+(aq) + 2F-(aq); Kf = 4.0 x 10-11
HF(aq) <----> H+(aq) + F-(aq); Ka = 7.2 x 10-4
Determine the molar solubilty of CaF2 in 0.20 M HF solution.
CaF2(s) <----> Ca2+(aq) + 2F-(aq); Kf = 4.0 x 10-11
Kf = [Ca+2][F-]^2
HF(aq) <----> H+(aq) + F-(aq)
0.20 0 0
0.20 - x x x
Ka = [H+][F-] / [HF]
7.2 x 10-4 = x^2 / 0.20 - x
x = 0.0116 M
[F-] = 0.0116 M
CaF2(s) <----> Ca2+(aq) + 2F-(aq);
S 0.0116
Ksp = [Ca+2][F-]^2
4.0 x 10-11 = S x (0.0116)^2
S = 2.97 x 10^-7 M
molar solubility = 3.0 x 10^-7 M
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