20.
Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of corresponding phenoxide ions.
The electron withdrawing groups are more effective in increasing the acidic strength at the para-position relative to the ortho position because of greater dispersal of charge on oxygen atom.
Resonance structure of phenoxide ion:

Resonance structure of paranitrophenol:
Resonance structures of ortho nitro phenol:

Thus, presence of nitro group at ortho and para position increase the acidic character.
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