The chemical reaction for nitrous acid in a solution with water is
HNO2 + H2O(l) ⇄ H3O^+ + NO2^-
Then
Ka = [H3O^+][NO2^-] / [HNO2]
If we let x represent the concentrations of [H3O^+] and [NO2^-] then
Ka = x^2 / [HNO2]
x = √(Ka * [HNO2])
x = √(4.5 × 10^-4 * 0.085)
x = 6.1846 x 10^-3 = [H3O+]
Therefore
pH = -log[H3O+]
pH = -log[6.1846 x 10^-3]
pH = 2.21
The answer to your question is 2.21.
