We investigate a few density functional theory-based reactivity indices of chemistry, with a view to arrive at an intercomparison and also consider their applications toward the problems of chemical significance. In particular, we propose to use the concepts of fugality and atom-atom polarizability to study the acidic strength of para-substituted benzoic acid derivatives. The nature of the variations and trends in the correlation of reactivity parameters and pK(a) values is shown to provide an insight into the applicability of these concepts to such reactions.