Answer:
4.70x10⁻³ = [H₃O⁺]
Explanation:
The Ka of benzoic acid is 6.3x10⁻⁵
The aqueous equilibrium of benzoic acid with its conjugate base could be written as:
HC₇H₃O₂(aq) + H₂O(l) ⇄ H₃O⁺(aq) + C₇H₃O₂(aq)
Where Ka is defined as:
Ka = 6.3x10⁻⁵ = [C₇H₃O₂] [H₃O⁺] / [HC₇H₃O₂]
As both H⁺ and C₇H₃O₂ are obtained from the same equilibrium:
[C₇H₃O₂] = [H₃O⁺]
And as [HC₇H₃O₂] = 0.35M we can write:
6.3x10⁻⁵ = [H₃O⁺] [H₃O⁺] / [0.35M]
2.205x10⁻⁵ = [H₃O⁺]²
4.70x10⁻³ = [H₃O⁺]
