Dry air mainly contains `O_(2)` and `N_(2)` in the ratio of nearly `1:4`by volume. Mol. Mass of dry air
`=x_(o_(2))M_(o_(2))+x_(N_(2))M_(o_(2)).M_(o_(2))=32,M_(N_(2))=32,M_(N_(2))=28.As M_(H_(2)O)=18` which is less than `M_(o_(2))` or `M_(N_(2))`, molar mass of moist air will be less and hence will have less density and will be lighter.
