Piperazine, an absorbent that has good potential for use as single amine or in mixtures with other amines, is studied in this work. Current measurement devices and methods are not able to measure the concentrations of all species that form during CO2 absorption into this amine and consequently calculation of equilibrium constants are not possible. To overcome this problem, using computational chemistry, optimized shapes of molecules and ions and needed energies of reaction and equilibrium constants are calculated. The eUNIQUAC model is used to model the behavior of the mixture. Results show the capability of the method used for this work and the power of computational chemistry to fill the gap between experimental data and data needed to build an equilibrium model.