Since the external pressure of the cylinder is 1 atm, the vapor pressure of the
gaseous A should be 1 atm. If conditions do not allow for the gaseous A to maintain
the pressure at 1 atm, the gaseous A cannot exist under the conditions. From the P-T
diagram given above, the gaseous A can exist up to 0.8 atm of its pressure at T,, and
hence the gaseous A cannot exist in equilibrium with the liquid A. Therefore all the
gaseous A will liquefy at 1 atm and T, .