Nitro groups have a large acid-strengthening effect on phenols. The _pK_a of 4-nitrophenol  is 7.15, whereas that of phenol is 9.89, a factor of almost 1000. Account for the greater acidity of 4-nitrophenol relative to phenol on the basis of resonance and inductive effects. Your answer will help explain why 2,4,6-trinitrophenol (also called picric acid) is so exceptionally acidic (_pK_a = 0.38) that it is more acidic than acetic acid (_pK_a = 4.76).