The formula for calculating pH is:
pH = -log [H+]
We calculate the value of [H+] using the given Ka value:
Ka = [H+]*[A-] / [HA]
Assuming that [H+] = [A-], therefore:
1.9 = [H+]^2 / 0.35
[H+] = 0.815
pH = -log [H+]
pH = -log 0.815
pH = 0.0886