Helium (initially at 160°F and 30 psia) is slowly withdrawn from a well-insulated rigid tank of volume 7 ft2 until the pressure reaches 18 psia. An electrical resistor inside the tank maintains the temperature at 160°F. Determine the mass of helium withdrawn (lbm) and the heat input to the resistor (Btu). Note that the problem can be worked without explicitly using values for enthalpy and internal energy.