A relative Gibbs free energy change approach is established in this work to estimate pK(a) values using density functional theory at the level of B3LYP/6-31+G(d,p) theory along with a polarizable continuum solvent model (PCM). A total of 101 pK(a) values for 90 carboxylic acids, 56 WC, values for both alcohols and phenols, and 51 pK(a) values for amines, which span over 16 orders of magnitude of experimental pKa (from 0.26 to 16.10) data, have been estimated in the aqueous condition. The pK, values obtained from this study are in good agreement with the experimental results with the standard deviation of 0.37 pK, unit for the carboxylic acids, 0.40 pK., unit for both alcohols and phenols and 0.52 pKa unit for amines. (C) 2011 Elsevier By. All rights reserved.