Total no of moles, ……….. (1)
Dividing by ,
Where, are mole fraction of species A, B,..
Total mass of the mixture,
Dividing by ,
Where, are mass fraction of species A, B, ..
(Figure 6.2)
= Molecular weight of mixture
Also, from ideal gas law, Substituting in (1),
……….. (1)
,
are mole fraction of species A, B,..
are mass fraction of species A, B, .. 
= Molecular weight of mixture 
