Question

An aqueous solution of barium nitrate is added dropwise to a solution containing 0.10 M sulfate ions and 0.10 M fluoride ions. The Ksp value for barium sulfate = 1.1 x 10-10 and the Ksp value for barium fluoride = 1.7 x 10-6

a.) Which salt will precipitate from solution first?

b.) What is the minimum [Ba2+] concentration necessary to precipitate the first salt? c.) What is the minimum [Ba2+] concentration necessary to precipitate the second salt?

c.) What is the minimum [Ba2+] concentration necessary to precipitate the second salt?

Answer 1

a)

First will be that which gets saturated first, so:

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

1.7*10^-6 = S*(0.10)^2

S = (1.7*10^-6) / ((0.10)^2) = 0.00017M = 1.7*10^-4 M of Ba+2

Ksp = [Ba+2][SO4-2]

1.1*10^-10 = S*(0.1)

S = 1.1*10^-10 / 0.1 = 1.1*10^-9 of Ba+2

Clearly,

BaSO4 solutoin precipitates FIRST, since it will not allow many Ba+2 ions in solution

b)

Min Ba+2 needed for precipitation of FIRST, BaSO4, salt:

S = 1.1*10^-10 / 0.1 = 1.1*10^-9 of Ba+2 M

c)

Min Ba+2 required:

S = (1.7*10^-6) / ((0.10)^2) = 0.00017M = 1.7*10^-4 M of Ba+2