The Vapor Phase Reference

As is customary we select the ideal vapor at unit pressure, P°, as the standard state. The partial molar free energy (chemical potential) of the vapor, p,(v), is 

P is the total pressure and f is the fugacity. The ratio f/P expresses the deviation of the gas from ideality (see Appendix 4.1). For the ideal gas reference state f0 = P° = 1. The correction for vapor non-ideality may be further expressed as  

B and C are second and third virial coefficients. The partial molar free energy of the vapor phase neglecting higher order terms is thus 

In Equation 5.5 pa°(v) is the standard state partial molar free energy of the vapor, P is the observed vapor pressure, and BP is a first order correction for vapor nonideality. 

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