Basics of Organic Chemistry

The acidity of the acids can be measured in terms of its pKa value, greater is the pKa value, weaker is the acid. The decreasing order of pKa values is
p-nitrophenol (pKa 7.15) > p-hydroxybenzoic acid (pKa 4.58)
> p-toluic acid (pKa 4.37)
> o-hydroxybenzoic acid (pKa 2.98)
The pKa value for o-hydroxybenzoic acid is the least due to ortho effect. Here both the groups are ortho to each other so due to steric repulsions it is the strongest acid among these.)