Question

" You would like to increase the solubility of CaF2 in water. What would you add to the solution:

NaF

HBr : Correct

CaCl2

NH3

KOH

Can someone explain why Her is the best answer, and why each other answer is wrong? Thanks!

Answer 1

CaF2(s) ⇋ Ca^2+ + 2F^-

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

What would happen if you increased the [F^-] by adding NaF, It's called the common ion effect; Due to this concentration of [F^-] increases and it will bind more and more to Ca^2+ leading to the formation of CaF2 (s) itself.

A similar reason is for CaCl2 but instead of F- here [Ca2+] will gonna increase eading to the formation of CaF2 (s) itself.

For a reason we won't go into here HF is a weak acid.

F^- + H2O ⇋ HF + OH^-

So adding the strong acid HBr will shift the equilibrium in favor of increased dissolution of CaF2.

H3O^+ + OH^- → 2H2O

Now I bet that was the last one you were thinking about?

Adding a base i.e NH3 and KOH they would push the equilibrium towards the solid, opposition to the acid.

Hope you got your answer.