Which one of the following gases is the lightest?

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Hydrogen

Helium

Oxygen

Nitrogen

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UPSC Geoscientist – 2022

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The lightness of a gas is determined by its molecular or atomic mass (for noble gases). Lighter gases have lower molecular/atomic masses and, at the same temperature and pressure, lower densities. Comparing the molar masses:
– Hydrogen (H₂): approx. 2.016 g/mol
– Helium (He): approx. 4.003 g/mol
– Oxygen (O₂): approx. 32.00 g/mol
– Nitrogen (N₂): approx. 28.01 g/mol
Hydrogen (H₂) has the lowest molar mass among the given options, making it the lightest gas.

– Gas density is directly proportional to its molar mass at constant temperature and pressure (Ideal Gas Law).
– Comparing molar masses allows identification of the lightest gas.

Hydrogen is the most abundant chemical substance in the universe. It is a highly flammable diatomic gas (H₂) under standard conditions. Helium is the second lightest element and is a noble gas, existing as a single atom (He). Oxygen and Nitrogen are the main components of Earth’s atmosphere, existing as diatomic molecules (O₂ and N₂).

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