Answer:
pH = 4.79
Explanation:
The equilibrium of benzoic acid, BH, is:
BH ⇄ B⁻ + H⁺
Where B⁻ is benzoate anion
Where the dissociation equilibrium, Ka, is:
Ka = 6.46x10⁻⁵ = [B⁻] [H⁺] / [BH]
If 20.0% is B⁻, 80.0% is BH, replacing:
6.46x10⁻⁵ = [20.0%] [H⁺] / [80.0%]
1.615x10⁻⁵ = [H⁺]
As pH = -log [H⁺]
pH = 4.79
