$1.$ Starting from the fundamental equation dg $=-$sdT $+$ dP and with the aid of the figure above,show or illustrate that at phase equilibrium $($saturated liquid and saturated vapor$)$ the equation below must be satisfied:${\displaystyle \underset{Ui}{\overset{Uv}{}}}$pdv $-$psat $($Uv$-$U$1)=0$

where U$,$ and U$,$ are the saturation vapor and liquid molar volumes at a given temperature, respectively.