Air is contained in the insulated cylinder shown in Fig. P6.70. At this point the air is at 20 lbf/in 2, 80 F, and the cylinder volume is 0.5 ft3. The piston cross-sectional area is 0.5 ft2, and the spring is linear with spring constant 200 lbf/in. The valve is opened, and air from the line at 100 lbf/in 2, 80 F, flows into the cylinder until the pressure reaches 100 lbf/in 2, and then the valve is closed. Find the final temperature.