The acetic acid will dissociate as
We will use Hendersen Hassalbalch's equation here
pH = pKa + log [salt] / [acid]
Where
[Acid] = concentration of protonated form
5.1 = 4.8 + log [salt] / [acid]
1.99= [salt] / [acid]
[salt] +[acid] = 0.5
1.99 [acid] + [acid] = 0.5
[acid] = 0.167 M
[acid] / total concentration = 0.167 / 0.5 = 0.33.4
So percentage = 33.4 % or approx 33 %
