In your refrigerator the working substance evaporates from liquid to vapor at -20 oC inside a pipe

Question:

In your refrigerator the working substance evaporates from liquid to vapor at -20 oC inside a pipe around the cold section. Outside (on the back or below) is a black grille inside which the working substance condenses from vapor to liquid at +40 oC. For each location find the pressure and the change in specific volume (v) if
a) The substance is R-12
b) The substance is ammonia