Thermodynamics - Piston Cylinder

[Icetray]

A piston-cylinder device initially contains 1.4-kg saturated liquid water at 200 C. Now heat is transferred to the water until the volume quadruples and the cylinder contains saturated vapour only. Determine:
(a) the volume of the tank,

-> Use P1 = P2 to solve -> find Vg at 200oC

(b) the final temperature and the pressure, and

P2 does not equal P1. Why?

I already have the solution for the above problem but I would like to know why for part b we don't have the final pressure = initial pressure.

I thought that all piston cylinder devices undergo isobaric processes.

Thanks in advance guys!

 

[jbsteele]

The answer is that the process is not isobaric because heat is being transferred to the water while its volume is increasing. This means that the pressure in the cylinder is changing as the temperature and volume of the water increase. As the temperature and volume of the water change, so does the pressure in the cylinder.