The following new thermodynamic equation is derived, e/i VI (d ln fi/dp)t = vt/n/rt, where VI is the mole fraction of the ith component in the charge fed to an equilibrium cell, vt is the total volume of the cell, n is the total number of moles in the cell, and fi is the fugacity of the ith component, with liquid and vapor in equilibrium. This equation does not contain any of the thermodynamic properties of the liquid phase. It can be used to test an assumed equation of state for the ability to calculate consistent fugacities for the vapor phase. For a two-component system, the derivatives, (d ln fi/dp)t, along the coexistence curve, can be determined from phase equilibrium data and used to calculate the molal volumes of the liquid and vapor phases.
Robert E. Barieau, Helium Research Center, Bureau of Mines Amarillo, Texas 79106