Helium gas (which may be treated as an ideal gas with R = 386 ft-lbf/lbm- degree R) is contained in a rigid 8-ft3 internal volume pressure vessel at an initial pressure of 600 psia and an initial temperature of 300 degree F. Some helium gas leaks out of the container until the final pressure is 400 psia and the final temperature is 185 degree F. Determine the mass {lbm} of helium gas that leaked from the container.