The rate of diffusion of methane at a given temperature is twice that of an unknown gas. The molar mass of the unknown gas is:

Correct answer: (c) 64

Explanation

By Graham's law of diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass:

• r₁/r₂ = √(M₂/M₁)

Methane (CH₄) has a molar mass of 16 g/mol and diffuses twice as fast as the unknown gas:

• 2 = √(M/16), so 4 = M/16 and M = 64 g/mol.

(Sulphur dioxide, SO₂, has this molar mass.)