 # Is humid air lighter than dry air?

In theory, moist air is lighter than dry air, because dry air is composed mainly of nitrogen (N2) and oxygen (O2) molecules, which weigh more than the water molecules (H20) in moist air. But what does Avogadro's law say about this?

## What is the molecular weight of water?

The weight of an individual atom is represented by its atomic weight. In this sense, in order to verify whether humid air is much lighter than dry air, we need to take the atomic weight of the atoms constituting it, namely :

• Hydrogen (H): 1.00784 u
• Oxygen (02): 15.999 u
• Nitrogen (H): 14.0067 u
• Carbon (C): 12.0107 u

If we know that the weight of a molecule is determined by adding the atomic weights of its atoms, then a water molecule (H2O) has a rounded molecular weight of 18 (1 + 1 + 16).

Based on this, the calculation is simple: free nitrogen (N2) has a round molecular weight of 28, and an oxygen molecule (O2) has a round atomic weight of 32. Therefore, a molecule of water is lighter than a molecule of nitrogen or oxygen. This means that, in theory, moist air is lighter than dry air, because :

• Water (H20) = 18.1558 u
• Nitrogen (N2) = 28.0134 u
• Oxygen (O2) = 31.998 u

## Avogadro's law of perfect gases

According to Amedeo Avogadro's law discovered in 1811, any fixed volume of a gas at constant pressure and temperature contains the same number of molecules. Regardless of the nature of the gas, the same number of molecules will exist in that volume. Therefore, to make a given volume of air more humid, we must add water vapour molecules to that volume.

The problem is that in order to add water molecules to the volume, we have to remove other molecules to keep the total number of molecules in the volume. Dry air consists mainly of nitrogen and oxygen molecules, which weigh more than water molecules.

This means that when a given volume of air is made more humid by the addition of water moleculesAs a result, heavier molecules are replaced by lighter ones. As a result, moist air is lighter than dry air if both are at the same temperature and pressure.