1st part: The Activity in Classical Thermodynamics -- Thermodynamic systems -- Gibbs and Helmhotz energies -- Escaping tendency and molar Gibbs energy -- Partial molar properties -- Chemical potential or partial molar Gibbs energy -- An overview of the notion of activity -- The concept of fugacity -- Ideal solutions -- Definitions of the activity -- Activity of a gaz -- Activities of non- electrolytes in solutions -- Activities of electrolytes: definitions -- Determination of the activity of non-electrolytes -- Determination of the activity of electrolytes -- Debye and Hückel 's relations -- Excess Gibbs energies and activities -- Chemical equilibrium constants : activities and Gibbs energies of reactions -- Derivation of thermodynamic equilibrium constants- pH and its measure -- General principles of calculations of ionic species concentrations in solutions involving activities -- 2nd part: The Activity in Statistical Thermodynamics -- Statistical thermodynamics in brief -- Concept of ensembles and postulates -- The canonical ensemble: notion of distribution -- Thermodynamic quantities in the framework of the canonical ensemble -- Other ensembles -- Systems of molecules and subsystems independent distinguishable and "indistinguishable" -- Perfect gases -- Classical statistical mechanics- Notion of configuration - Classical canonical partition function -- The configurational partition function - Molecular distribution functions -- The radial distribution function -- Radial distribution function and thermodynamic quantities- calculations of the internal energy and of the pressure of a system -- Radial distribution function and calculation of the isothermal compressibility coefficient of a system -- The chemical potential and the radial distribution function: General formal introduction of the activity and of the activity coefficient -- Imperfect gases - The virial coefficients in terms of interaction potential energies -- A statistical expression of the activity: A relation between it and the concentration in the case of an imperfect gaz -- Activities of gases in a mixture of imperfect gases -- Chemical equilibria between gases and statistical thermodynamics -- Activity coefficients of gases -- Activities and concentrations of non-electrolytes in dilute liquid solutions - McMillan- Mayer's theory of their osmotic pressure -- A study of dilute solutions of non-electrolytes at constant pressure and temperature -- The activity coefficients of solutes in binary non-electrolyte solutions -- Molecular distribution functions in binary mixtures -- The Kirkwood-Buff's theory- Changes of chemical potentials of solutes with their concentrations at constant pressure and temperature -- Expressions of the chemical potentials of the components of ideal solutions of non-electrolytes.