A cylinder is fitted with a 4-in.-diameter piston that is restrained by a linear spring (force proportional to distance). The spring force constant is 400 lbf/in and the piston initially rests on the stops, with a cylinder volume of 60 in 3. The valve to the air line is opened and the piston begins to rise when the cylinder pressure is 22 lbf/in 2. When the valve is closed, the cylinder volume is 90 in 3 and the temperature is 180 F. What mass of air is inside the cylinder?