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

1 Answer
May 18, 2017

Answer:

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#.