WAEC 2023 Chemistry · Question 20

The gas that is less dense than air is

Options

A.nitrogen
B.chlorine
C.Carbon(IV) oxide
D.Oxygen

Answer: A. nitrogen

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We need the gas that is less dense than air. Density of a gas is proportional to its molar mass (at the same temperature and pressure), so we compare molar masses with the average molar mass of air. Step 2: Concept Air is about 78% nitrogen, 21% oxygen, and 1% other gases. Its average molar mass is about 28.8 g/mol (often rounded to 29). A gas with a molar mass lower than about 29 is less dense than air. A gas with a higher molar mass is denser. Step 3: Compare the options | Gas | Formula | Molar mass (g/mol) | Compared with air (≈29) | |---|---|---|---| | Nitrogen | N₂ | 28 | Slightly lighter | | Chlorine | Cl₂ | 71 | Denser | | Carbon(IV) oxide | CO₂ | 44 | Denser | | Oxygen | O₂ | 32 | Denser | Step 4: Answer Only nitrogen (M = 28) has a molar mass below that of air (about 29), so it is slightly less dense than air. Tip: Memorize the "reference point": air ≈ 29. Any common gas with a molar mass below 29 (H₂, He, NH₃, CH₄, N₂, CO, C₂H₂) is lighter than air, and gases above 29 (O₂, CO₂, Cl₂, SO₂, HCl) are heavier. This also tells you how to collect gases: lighter-than-air gases by downward displacement of air, and heavier ones by upward displacement.

AI can make mistakes. Report anything that looks wrong.