Q. Which of the following gasses is/are lighter than air ?
1. Hydrogen chloride
2. Sulfur dioxide
3. Helium
Select the correct answer from the code given below:
a) 1 and 2
b) Only 3
c) 2 and 3
d) Only 1
Correct Answer: b) Only 3
Question from UPPSC Prelims 2025
Explanation:
A gas is lighter than air if its molar mass (or density at the same conditions) is less than that of air (~29 g/mol; density ~1.225 g/L at STP).
– Hydrogen chloride (HCl): Molar mass ≈ 36.5 g/mol; density ≈ 1.49 g/L. Heavier than air → not lighter.
– Sulfur dioxide (SO2): Molar mass ≈ 64 g/mol; density ≈ 2.93 g/L. Heavier than air → not lighter.
– Helium (He): Molar mass ≈ 4 g/mol; density ≈ 0.1785 g/L. Much lighter than air → lighter.
Thus, only Helium is lighter than air.
