That implies that the number of molecules in a given container won't change, when you change only the type of gas filling the container. (This is one way to state Avogadro's Law).
Here is the same very small container, filled with dry air on the left, and wet air on the right. The drawings aren't to scale; the distance between molecules is much, much larger under normal conditions. Notice that in both pictures, there are 10 molecules in the container.
The molecular weights of O2 (in red), N2 (in blue) and H2O are about 32, 28, and 18 atomic mass units, respectively. Substituting a water molecule for either an oxygen or a nitrogen molecule, then, will decrease the total mass of air in the container. Since the volume of the container is the same in both cases, the density of water-containing air must be less than the density of dry air.
Temperature and pressure affect the distance between molecules, so in making this comparison, we've assumed that both the dry and wet air are at identical temperatures and pressures.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/gases/faq/print-wet-air-dry-air-density.shtml