Which of the following gasses is/are lighter than air?

Question

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:

1. A. 1 and 2
2. B. Only 3 (Correct answer)
3. C. 2 and 3
4. D. Only 1

Correct Answer

Option B — Only 3

Detailed Solution & Explanation

The correct answer is Only 3.

Key Points

• A gas is lighter than air if its molar mass is less than about 29 g/mol, the average molar mass of air.
• 3. Helium — molar mass 4. Far lighter than air. ✓
• 1. Hydrogen chloride — molar mass 36.5. Heavier than air. ✗
• 2. Sulphur dioxide — molar mass 64. Much heavier than air. ✗
• Only helium qualifies, giving option (b).

Additional Information

• Gases lighter than air: hydrogen (2), helium (4), methane (16), ammonia (17), neon (20) and carbon monoxide (28).
• Gases heavier than air: oxygen (32), carbon dioxide (44), chlorine (71), sulphur dioxide (64) and hydrogen sulphide (34).
• Helium is used in balloons and airships in preference to hydrogen because it is non-inflammable — the lesson of the Hindenburg disaster (1937).
• Helium is also used in deep-sea diving cylinders mixed with oxygen, since it is far less soluble in blood than nitrogen and so avoids decompression sickness (the bends).