Question

1. What is the pH of a solution of CaF2 dissolved in H2O? Ignoring activity

2. What is the solubility of CaF2 in aqueous solution? Ignoring activity

Ignore water autoprotolysis vs. Account for water autoprotolysis <- what is the difference?

3. What is the solubility of CaF2 in at pH 10? Ignoring activity.

Calculation process and the answer of these questions?

why number 3 do not need charge balance like number 1 and 2?

Thank you!

Answer 1

1)

CaF2 ---------------> Ca+2 + 2 F-

                                S          2S

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

3.45 x 10^-11 = 4S^3

Molar solubility S = 2.05 x 10^-4 M

F- = 2 x 2.05 x 10^-4 = 4.1 x 10^-4 M

HF Kb valuee = 1.59 x 10^-11

F-    +    H2O   -----------------> HF    +      OH-

4.1 x 10^-4                                 0                 0

4.1 x 10^-4 - x                             x                 x

Kb = x^2 / 4.1 x 10^-4 - x

1.59 x 10^-11 = x^2 / 4.1 x 10^-4 - x

x = 8.066 x 10^-8

[OH-] = x = 8.066 x 10^-8

pOH = 7.09

pH = 6.91

Molar solubility S = 2.05 x 10^-4 M