A 35 ft3 rigid tank has air at 225 psia and ambient 600 R connected by a

Question:

A 35 ft3 rigid tank has air at 225 psia and ambient 600 R connected by a valve to a piston cylinder. The piston of area 1 ft2 requires 40 psia below it to float, Fig. P3.99. The valve is opened and the piston moves slowly 7 ft up and the valve is closed. During the process air temperature remains at 600 R. What is the final pressure in the tank?