Gas which obeys the equation of state \(p\, V=n\, R\, T\) (the ideal gas law; \(p\) is the pressure, \(V\) the volume, \(n\) the amount of molecules, \(R\) the gas constant and \(T\) the thermodynamic temperature). For an ideal gas the fugacity, \(f\), of each constituent B is equal to its partial pressure, \(p_{\rm{B}}\), \(f_{\rm{B}}=p_{\rm{B}}=x_{\rm{B}}\ p\), where \(x_{\rm{B}}\) is the amount fraction of B.