( atm = . × Pa) occupies a volume of . litres. This universal volume is called molar volume.
Thus the cylinder in this example contains mol of helium. Further, since helium is monatomic, its predicted (and observed) molar specific heat at constant volume, C v = ( / ) R , and molar specific heat at constant pressure, C p = ( / ) R + R = ( / ) R . Since the volume of the cylinder is fixed, the heat required is determined by C v . Therefore,