Question

The Ksp of barium fluoride is 1.00 x 10^–6. The Ksp of calcium fluoride is 3.90 x 10^–11. An aqueous solution of sodium fluoride is slowly added to a water sample that contains barium ion (3.95×10^-2M ) and calcium ion (4.65×10^-2M ). What is the remaining concentration of the first ion to precipitate when the second ion begins to precipitate?_____M

Answer 1

The Ksp of BaF2 = 1.00 x 10^–6.

The Ksp of CaF2 = 3.90 x 10^–11.

here the Ksp of CaF2 < Ksp of BaF2.

so CaF2 first begins to precipitate. BaF2 is second begins to precipitate.

BaF2   ----------------> Ba+2 + 2F-

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

1.00 x 10^-6 = [3.95 x 10^-2] x [F-]^2

[F-] = 5.03 x 10^-3 M

CaF2   -----------> Ca+2   + 2F-

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

3.90 x 10^-11 = [Ca+2][5.03 x 10^-3]^2

[Ca+2] = 1.54 x 10^-6 M

remaining concentration = 4.65 x 10^-2 - 1.54 x 10^-6

remaining concentration of [Ca+2] = 4.65 x 10^-2 M