A mass of 39 lbm of helium undergoes a process from an initial state of 50 ft3/ lbm and 60°F to a final state of 20 ft3/lbm and 240°F. Determine the entropy change of helium during this process assuming the process is reversible. The gas constant of helium is R = 0.4961 Btu/lbm·R. The constant volume specific heat of helium at room temperature is cv = 0.753 Btu/lbm·R. The entropy change of helium during this process is Btu/R.