Question

Why do solutions of barium chloride, and barium fluoride give rise to solutions with different `pH` values?

Answer 1

Because `"fluoride ion"` undergoes hydrolysis in aqueous solution.

Explanation:

Consider the solubility equilibrium for `BaF_2`:

`BaF_2(s) rightleftharpoonsBa^(2+) + 2F^-`

Fluoride is moderately basic.............

`F^(-) + H_2O rightleftharpoons HF(aq) + HO^-`

Addition of `H_3O^+` will drive these equilibria to the right as written, and MORE barium salt will go into solution.

On the other hand, `"chloride ion"`, A WEAK BASE, does not speciate in solution, and is inert to added acid.

Note that had we `K_"sp"` data for `BaF_2(s)`, and `K_a` data for `HF` we could solve the solubility equilibrium precisely, and develop an expression for solubility that depends on `pH`.