At Point A:
x = vapor fraction
1-x = liquid fraction

Point A represents a mixture of saturated liquid and vapor at pressure, PA. The enthapy at A, HA, is a weighted average of the enthalpies of the saturated phases:

This is the same formula used to balance a seesaw.

Hence the name "lever rule." The lever rule can also be used for other properties besides H, like volume.


At 25°C, the specific volume of saturated liquid is 1.003 cm^3/g and the volume of saturated vapor is 43400 cm^3/g. What is the volume of a mixture with vapor fraction, x=0.3?


The lever rule calculation is similar to that for calculating enthalpy.

VA = xVV + (1-x)VL

VA = (0.3)(43400) + (1-0.3)(1.003)

VA = 13021 cm^3/g

Notice that the saturated liquid's contribution to the volume is almost negligible compared to the contribution of the vapor.

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