Question

Barium and Calcium ions in aqueous solutions can be separated by the differences in their solubilizes as fluoride salts( Baf2, Ksp=1.8E-7 and CaF2, ksp=5.3E-11. a) which salt precipitates first when slowly adding fluoride ion to a solution that is .1 M In both Ba and Ca? Explain. b) wat concentration of fluoride ion will precipitate the maximum amount of the first salt from the solution described in "part a" while not precipitating the second? Explain.

Answer 1

Baf2, Ksp=1.8E-7 and CaF2, ksp=5.3E-11

Clearly, Ksp for CaF2 << BaF2, therefore

CaF2 saturates first, therefore, preicptates first

b)

find concentraiton of F- when the other specie sprecipitet ( BaF2)

KSp = [Ba+2][F-]^2

1.8*10^-7 = (0.1)[F-]^2

[F-] = sqrt((1.8*10^-7/0.1)) = 0.001341 M

then... get [Ca+2] left

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

5.3*10^-11 = [Ca+2} (0.001341)^2

[Ca+2] = (5.3*10^-11)/(0.001341^2) = 0.0000294

[Ca+2] precipitated = 0.1 -0.0000294 = 0.0999M

[Ca+2] = precipitated = 0.0999 M, [Ca+2] left = 0.0000294 M