Universal Gas Constant:

Real gases below critical pressure and above the critical temperature
obey perfect gas law which is a combination of Boyle’ s and Charle’ s law.

Boyle’ s law states that for constant temperature, the density varies directly as absolute pressure.

Charle’ s law states that for constant pressure, the volume of a given mass
of gas varies as its absolute pressure.

Thus, pv = mRT

On molal basis, the above equation is expressed as

1. Isothermal process: An isothermal process in the one in which temperature
remains constant.

i.e. pv = Constant.

2. Adiabatic Process: An adiabatic process in the one in which there is no heat transfer
to and from the gas. i.e. pvr = Constant,


where r is the ratio of specific heat of gas at constant pressure and constant volume.= Cp/Cv, 1.4 for air.