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

May 18, 2017

Because $\text{fluoride ion}$ undergoes hydrolysis in aqueous solution.

#### Explanation:

Consider the solubility equilibrium for $B a {F}_{2}$:

$B a {F}_{2} \left(s\right) r i g h t \le f t h a r p \infty n s B {a}^{2 +} + 2 {F}^{-}$

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

${F}^{-} + {H}_{2} O r i g h t \le f t h a r p \infty n s H F \left(a q\right) + H {O}^{-}$

Addition of ${H}_{3} {O}^{+}$ will drive these equilibria to the right as written, and MORE barium salt will go into solution.

On the other hand, $\text{chloride ion}$, A WEAK BASE, does not speciate in solution, and is inert to added acid.

Note that had we ${K}_{\text{sp}}$ data for $B a {F}_{2} \left(s\right)$, and ${K}_{a}$ data for $H F$ we could solve the solubility equilibrium precisely, and develop an expression for solubility that depends on $p H$.