we know that:
Ka = [H+][OCl-] /[HOCl]
and HOCl dissociates into equal number of ions H+ = OCl-
We substitute [OCl-] for [H+}
Hence
Ka = [H+][H+] /{HOCl]
Ka = [H+]^2 / [HOCl]
[H+]^2 = Ka X [HOCl]
[H+] = sqrt{Ka X [HOCl]}
[H+] = sqrt{ 3.5 x 10^-8 x 0.10}
[H+] = sqrt[ 3.5 x 10^-9
[H+] = 5.916 x 10^-5
you know: pH = -log(10) [H+]
pH = -log(10) 5.916 x 10^-5
pH = -(-4.227965)
pH = 4.227965
pH = 4.23 .
