QQuestion
State Avogadro’s law. Use simple calculation to show that moist air is lighter than dry air. [1+2]
✓Answer
Avogadro’s Law : Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.
To show that moist air is lighter than dry air
According to Avogadro’s Law, equal volumes of gases contain an equal number of molecules.
Molecular mass of dry air ≈ 29
Molecular mass of H₂O = (2 × 1) + 16 = 18
When water vapour mixes with dry air, some of the heavier air molecules (average molecular mass = 29) are replaced by lighter water vapour molecules (molecular mass = 18).
Since 18 < 29, the average molecular mass of moist air decreases.
Therefore, moist air is lighter than dry air.
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