would anyone happen to know what the expression for an isothermal change in Gibbs free energy for a liquid is when the pressure changes from pi to pf. Also an expression for when its a perfect gas? i cant seem to figure them out.. and how would they be deduced? i've come across one equation: Gf=Gi +nrtln(pf/pi) <- that i think may be for perfect gas. but i'm still unsure abotu the first expression. thanks for any help.