How would you define or calculate entropy in a twophase liqu

How would you define or calculate entropy in a two-phase (liquid-vapor) region? In a compressed liquid?

Solution

Let entropy of saturated liquid be sf, entropy of saturated vapor be sg and quality be x.

Then entropy in two-phase region is given by s = sf + x*(sg - sf)

In a compressed liquid, we can use the relationship

s2 - s1 = Cp ln (T2 / T1) - R ln (P2 / P1)

Here state 1 can be taken as the saturated liquid and then entropy at state 2 where P2, T2 is given can be evaluated.