Taking `N_(2) and O_(2)` as the main components of air (79% `N_(2)`, 21% `O_(2)` by volume), find the average molecular mass of air.

Explanation

For a mixture of different gases, the average molecular mass of the mixture is taken as-
Average molecular mass`=sumx_(i)M_(i)=x_(N_(2))M_(N_(2))+x_(O_(2))M_(O_(2))` where `x_(N_(2))` and `x_(O_(2))` are mole fractions of `N_(2) and O_(2) and M_(N_(2)) and M_(O_(2))` are their molecular masses.
As the same temperature and pressure, equal volumes of all gases contain equal number of moles, their molar ratio is same as the ratio of their volumes.
`therefore x_(N_(2))=0.79,x_(O_(2))=0.21` . also `M_(N_(2))=28u,M_(O_(2))=32u`
`therefore` Average molecular mass of air`=(0.79xx28+0.21xx32)u`
`=(22.12+6.72)u=28.84u`.