Suppose S is the molar solubility of silver benzoate in water, then
C6H5COOAg(s) ⇌ C6H5COO(aq)–+Ag(aq)+
Ksp = S2 ∴ S = √2.5×10−13 = 5.0 × 10−7M
If the solubility of salt of weak acid of ionization constant Ka is S′, then Ksp,
Ka and S′ are related to each other at pH = 3.19.
∴ [H+] = 6.46 × 10–4M
Ksp = S′2[

Ka

Ka+[H+]

] , S′ =
S′ = [

2.5×10−13×7.106×10−4

6.46×10−5

]

1

2

= (2.75×10−12)

1

2

= 1.658 × 10−6M
∴ The ratio

S′

S

=

1.658×10−6

5.0×10−7

= 3.32
Silver benzoate is 3.32 times more soluble in buffer of pH 3.19 than in pure water.