The acidity of phenol arises from the loss of H of O – H group as $\mathrm{H}^{+}$ . This loss is facilitated if the polarity of the O – H bond increases and H carries substantial $\delta^{+}$ charge. In (I), the presence of the highly electron withdrawing $\mathrm{NO}_2$group at the ortho position increases the polarity of the O – H bond.This helps in release of H$^{+}$ and increases acidity. (II) is less acidic because $\mathrm{-OCH}_3$ is an electron releasing group and increases the e$^{-}$ density on O of OH and H$^{+}$ release is suppressed. This can be understood from the resonance structures of II which are