Molecular Weights and Gas Constants for Dry Air and Water Vapour:

From the respective mole fractions & molecular masses of component gases, the molecular mass of the dry air portion might be computed.

Since a portion of volume represents the mole fraction. By using the values for mole fraction from given Table:  We contain molecular Mass of dry air

M_a = ∑ My                                                                             ... 6.9

28.02 (0.7803) + 32 (0.2099) + 39.91 (0.0094) + 44(0.0003) + 2.02 (0.0001)

= 28.966

       Composition of Dry Part in Atmospheric Air

By knowing that the value of the universal gas constant is 8.3143 kJ/kg mole K, the gas constants used for the two parts of moist air are as following:

Dry air

 M_a= 28.966

R = 8.3143/ 28.966

 = 0.2871 kJ/kg. K

 Water vapour

 M_v= 18.016

R = 8.3143/18.016

=0.461 kJ/kg. K

here subscript v refers to water vapour.