(1) In p-nitrophenol, nitro group (NO2) is an electron withdrawing group present at para position which enhances the acidic strength (-1 effect). The O-H bond is under strain and release of proton (H+) becomes easy. Further p-nitrophenoxide ion is more stabilised due to resonance.
(2) Since the absence of electron withdrawing group (like – NO2) in phenol at ortho and para position, the acidic strength of phenol is less than that of p-nitrophenol.
