W =-δ n (gas) RT

δU = Q+W

δS =δH/T

δA =δU-tδS，

δG =δH-tδS .

(2) Initial state: 373K, 130kPa liquid water; Final state: 373K, 130kPa steam.

From the initial state to 373K, 100kPa liquid water, Gibbs free energy becomes δ G (1).

From 373K, 100kPa liquid water to 373K, 100kPa water vapor, Gibbs free energy changes to δ g (2) = 0 (reversible phase transition).

From 373K, 100kPa water vapor to the final state: 373K, 130kPa water vapor, Gibbs free energy becomes δ G (3).

δG =δG( 1)+δG(2)+δG(3)

= v δ p+0+nrtln (130/100) v = molar mass divided by density, in cubic meters.