Question

Which is denser at the same temperature and pressure, dry air or air saturated with water vapor? Explain.

a) Dry air
b) Air saturated with water vapor
c) They have the same density
d) It depends on the specific conditions

Answer 1

Air saturated with water vapor is less dense than dry air at the same temperature and pressure because water vapor has a lower molecular weight than the average molecular weight of dry air.

Step-by-step explanation:

The density of air changes with the incorporation of water vapor due to the fact that the molecular weight of water vapor (18.02 g/mol) is less than the average molecular weight of dry air (approximately 29 g/mol).

At the same temperature and pressure, air that is saturated with water vapor will be less dense than dry air, because the lighter water vapor molecules replace the heavier nitrogen and oxygen molecules.

This can be understood in terms of the ideal gas law, where the density (ρ) is directly proportional to molecular weight (M) for the same pressure (P) and temperature (T), according to the equation ρ = PM / RT (where R is the universal gas constant).