Which of the following gases has the lowest rate of diffusion? [H=1.0, C=12.0, N = 14.0, O= 16.0]

Analyze the question: We need the gas that diffuses most slowly. This is a Graham's Law question.

Concept: Graham's Law of Diffusion says the rate of diffusion of a gas is inversely proportional to the square root of its molar mass (or density):

Rate ∝ 1/√M

So the heavier the gas, the slower it diffuses. The lightest gas diffuses fastest.

Calculate the molar masses:

• A. Nitrogen (N₂) = 2 × 14 = 28 g/mol
• B. Methane (CH₄) = 12 + (4 × 1) = 16 g/mol
• C. Ammonia (NH₃) = 14 + (3 × 1) = 17 g/mol
• D. Oxygen (O₂) = 2 × 16 = 32 g/mol

Compare: Oxygen has the highest molar mass (32 g/mol), so it has the lowest rate of diffusion. The order of rates from fastest to slowest is:

CH₄ (16) > NH₃ (17) > N₂ (28) > O₂ (32)

Tip: In "lowest rate of diffusion" questions, just compute the molar masses. Highest molar mass means slowest, and lowest molar mass means fastest. Remember that nitrogen and oxygen are diatomic (N₂, O₂), so don't forget to double their atomic masses. That is a common trap in this question!