[AhmedEzatAlzawalaty]

which is more acidic:p-nitrobenzoic acid or phthalic acid
and which is more acidic 3-nitrophenol or 4-nitrophenol?

[Yggdrasil]

Draw the resonance structures of each deprotonated acid.  Does the extra group stabilize the negative charge or destabilize it?

[Ahmed Abdullah]

I think  3-nitrophenol is more acidic than 4-nitrophenol. Nitro radical is a deactivating agent, Since in 3-nitrophenol NO2 is attached to electron rich para site it will able to stabilize the conjugate base more than being at ortho position(which is deficient in electron cloud).

[Yggdrasil]

You are correct that a nitro group is a deactivating agent.  If you draw out the resonance structures, you will see that the nitro group deactivates (i.e. draws electron density away from) the ortho- and para- positions more than the meta-positions.  Therefore, a nitro group would stabilize a negative charge at the ortho- or para- positions more than it would stabilize a negative charge at the meta- position.  Therefore, 4-nitrophenol (i.e. para-nitrophenol) is more acidic because the acidic OH group is para to the nitro group, whereas the nitro is meta to the nitro group in 3-nitrophenol (i.e. meta-nitrophenol)

[Ahmed Abdullah]

Sorry, I meant para-nitrophenol by 3-nitrophenol accidently.