Any thermodynamic property, , can be expressed as the sum of an ideal gas contribution and a residual non-ideal contribution: , where and are the ideal gas and residual contributions, respectively. For a given equation of state, the residual contribution can then be expressed as a function of , , and compressibility factor . In this Demonstration, the compressibility factor for a single gas chemical species (acetylene) is computed, from which the enthalpy ( in ) and entropy ( in can be determined for given and . You can select from one of three cubic equations of state (Redlich–Kwong, Soave–Redlich–Kwong, or Peng–Robinson) as well as the temperature (in ) and the pressure (in ). The reference state is taken an ideal gas at and . This information is then used to obtain the molar volume (in ) as well as additional thermodynamics properties such as the Gibbs free energy ( in ), Helmholtz free energy ( in ), and internal energy ( in ). In addition, is plotted versus reduced pressure for a user-specified reduced temperature (), where and are the critical pressure and temperature for acetylene. For an ideal gas .