Why dry air is heavier than moist air?

Explanation

Dry air mainly contains gases like nitrogen and oxygen along with small proportions of the other gases. The wet air on the other hand, also contains a small amount of water vapour. Now, vapour density of water vapours is less than that of nitrogen and oxygen because molar mass of `H_(2)O` (18) is less than that of `N_(2)(28)` and `O_(2)(32)`. This means that hevier molecules of nitrogen and oxygen have been replaced by lighter molecules of water vapours when the dry air changes to wet air. Therefore dry air is heavier than wet air.