Account for the following: The pKa value of 4-nitrobenzoic acid is lower than that of benzoic acid.

pKa value of 4-nitrobenzoic acid (3.41) is lower than that of benzoic acid (4.19). Lower is the pKa value, greater is the acid strength. The electron withdrawing nitro (-NO2) group increases the acidity of benzoic acid.