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A 4:1 molar mixture of helium (He) and methane (CH4) is kept in a vessel at 20 bar pressure. Due to a small hole in the vessel, the gas mixture leaks out. What is the molar composition of the gas mixture effusing out initially?

1. 8: 1
2. 4: 1
3. 1: 4
4. 4: 3

Correct answer: 8: 1

Solution

By Graham's law, the rate of effusion of each gas is proportional to (mole fraction) / sqrt(molar mass). He: M=4, CH4: M=16. Ratio = (4/5 * 1/sqrt(4)) / (1/5 * 1/sqrt(16)) = (4 * 1/2) / (1 * 1/4) = 2 / 0.25 = 8. So effusing ratio He:CH4 = 8:1.

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