Which one of the following gases is lighter than air?

The correct option is Hydrogen.

Step 1: Understand Molecular Weights and Density of Air

To determine whether a gas is lighter or heavier than air, we compare its molar mass (molecular weight) to the average molar mass of dry air.

The average molar mass of dry air is approximately 29 g/mol (composed mainly of Nitrogen, N₂ ≈ 78%, and Oxygen, O₂ ≈ 21%).

Step 2: Analyze the Molar Masses of the Given Gases

1. Carbon dioxide (CO₂):
Molar mass = 12 + (2 × 16) = 44 g/mol.
Since 44 g/mol > 29 g/mol, carbon dioxide is heavier than air.

2. Chlorine (Cl₂):
Molar mass = 2 × 35.5 = 71 g/mol.
Since 71 g/mol > 29 g/mol, chlorine is significantly heavier than air.

3. Oxygen (O₂):
Molar mass = 2 × 16 = 32 g/mol.
Since 32 g/mol > 29 g/mol, oxygen is slightly heavier than air.

4. Hydrogen (H₂):
Molar mass = 2 × 1 = 2 g/mol.
Since 2 g/mol < 29 g/mol, hydrogen is extremely lighter than air.

Conclusion:

Hydrogen has the lowest molecular mass (2 g/mol) among all the given options, making it significantly lighter than air.