Why para nitro benzoic acid is less acidic than meta nitro benzoic acid?
Answers
First, consider the chemical environment near the dissociating hydrogen: the hydrogen is a proton near the electron cloud around the oxygen. Now consider how the density of the electron cloud affects the hydrogen - oxygen bond: because the hydrogen nucleus is attracted to the electron cloud, a more dense electron cloud means a stronger hydrogen-oxygen bond (and thus, a weaker acid), while a less dense electron cloud means a weaker hydrogen-oxygen bond (and thus, a stronger acid). Now onto the problem:
In nitrobenzoic acid, the nitro group is quite electronegative and would naturally draw electrons away from the other atoms in the bond. In meta nitro benzoic acid, the nitro group is closer to the acid group than in para nitro benzoic acid, so the nitro group does a better job drawing electrons away from the acid group in meta nitro benzoic acid than in para nitro benzoic acid. Thus, the acid group in para nitro benzoic acid has higher electron cloud density than the acid group in meta nitro benzoic acid, and, by logic in the first paragraph, para nitro benzoic acid is the weaker acid.