The general principles outlined in Chap. 2 and therein specifically employed for a rigorous statistical mechanical description of thermodynamics are further developed in this chapter for treating the central issue of Chemistry: the thermodynamics of the chemical transformations, either in vapor or in condensed phase. After an initial introduction to chemical equilibriumChemical equilibrium, with specific emphasis on the chemical potentialChemical potential and the related concept of activity, the chapter will illustrate the treatment of the main determinants of whatever problem concerning the chemical equilibriumChemical equilibrium: the partial molecular propertiesPartial molecular properties and the equilibrium constantEquilibrium constant. In the final part of the chapter two examplesExamples are reported: the first one dealing with the calculation of the pKa, i.e., the equilibrium constantEquilibrium constant for a proton transfer reaction of a Bronsted acid to water in water solution, and the second one illustrating the conformational equilibriumConformational equilibrium of a simple biomolecule in water solution.