The acidity constants of some carboxylic acids in aqueous solution have been calculated. The calculations were carried out using Moller-Plesset (MP) perturbation theory. The Polarizable Continuum Model (PCM) is used to describe the solvent. This model furnishes PKA values that agree more closely with experimental data than those obtained at the level of Hartree-Fock (HF) and Density Functional Theory (DFT-B3LYP). The root-mean-square of errors of the calculated PKA values are less than 1.0 for the studied acids. The molecules analyzed consist of acids with PKA values in the range of 1.30 to 5.05, and have been partitioned into three classes. Class I includes acids with PKA values higher than 4.00. Class II includes strong acids with PKA values between 3.00 to 4.00. Class III includes very strong acids with PKA values less than 3.00. The calculated PKA values for the acids in Class I
and Class II agree more closely with experimental values. The root-mean-square of errors for the Class I and Class II compounds are 0.70 and 0.78 PKA units, respectively.