Question

Precipitation

Learning Goal:

To understand the relationship between precipitation and the solubility product and to be able to predict whether a substance will precipitate or not.

Precipitation is the formation of an insoluble substance. For the equation AB(s)⇌A+(aq)+B−(aq) , precipitation represents a shift to the left and the production of a solid. From Le Châtelier's principle, we know that when the product of the concentrations of A+ and B− gets above a certain level, the reaction will respond by shifting left to decrease the concentrations of A+ and B− . This critical level, Ksp , is a constant at a certain temperature. In this case, Q=[A+][B−] . where Q is the ion product.

When Q>Ksp , precipitation occurs until Q is equal to Ksp .

When Q≤Ksp , a precipitate does not form.

Part A - Calculate the value of Q What is the value of Q when the solution contains 2.00×10−3 M C a 2+ and 3.00× 10 −2 M S O 4 2− ? Express your answer numerically.

Part C What concentration of the barium ion, B a 2+ , must be exceeded to precipitate Ba F 2 from a solution that is 1.00× 10 −2 M in the fluoride ion, F − ? K sp for barium fluoride is 2.45×10−5 . Express your answer with the appropriate units.

Answer 1

CaSO4   ----------------> Ca+2 + SO42-

Q = [Ca+2][SO42-]

   = (2.00×10^−3)(3.00× 10^ −2)

Q = 6.00 x 10^-5

Ksp = 4.93 x 10^-5

here Q > Ksp .so precipitate forms

Part C)

BaF2   ---------------> Ba+2 + 2F-

                                   Ba+2    1 x 10^-2

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

2.45×10^−5 = [Ba+2](1 x 10^-2)^2

[Ba+2] = 0.245 M