Answer:
4.27
Explanation:
Let's consider the dissociation of a generic monoprotic acid.
HA(aq) → H⁺(aq) + A⁻(aq)
The pH is 2.36. The concentration of H⁺ is:
pH = -log [H⁺]
[H⁺] = antilog -pH
[H⁺] = antilog -2.36 = 4.37 × 10⁻³ M
We know that the concentration of the acid Ca = 0.3535 M. We can find the acid dissociation constant using the following expression.
[H⁺] = √(Ca × Ka)
Ka = [H⁺]²/Ca
Ka = (4.37 × 10⁻³)²/0.3535
Ka = 5.40 × 10⁻⁵
The pKa is:
pKa = -log Ka = -log 5.40 × 10⁻⁵ = 4.27
