The presence of the Nitro group in the ortho or para position decreases the electron density in the −OH bond. As a result, it is easier to lose a proton from mono substituted nitro phenol Hence, ortho and para nitrophenols are stronger acids than phenol.
The acidic nature of nitrophenols increases in the order: meta < ortho < para.
Meta position shows only -I effect. Hence, it is less acidic than the para and ortho isomers.
Between para and ortho nitrophenols, para is more acidic due to the presence of an intramolecular hydrogen bonding that locks hydrogen of −OH bond in ortho isomer.