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Graham's law of effusion
Intermediate
Rate of effusion is inversely proportional to the square root of molar mass.
The rate of effusion through a tiny hole is inversely proportional to √M. Light gases escape faster. It was used to separate uranium isotopes.
H₂ effuses 4 times faster than O₂.
Valid when the hole is smaller than the mean free path.
In more detail
Graham's law states that the rate of effusion of a gas through a small hole is inversely proportional to the square root of its molar mass. Lighter gases escape faster.
Real-world uses
- Isotope separation historically used effusion of uranium hexafluoride.
- Helium balloons deflate because helium effuses through the rubber.
- Engineers use Graham's law to separate light gases such as helium and hydrogen by how quickly they leak through a porous barrier.
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Frequently asked questions
What is effusion?
The escape of gas molecules through a small opening into a vacuum.
Which gas effuses fastest?
Hydrogen, because it has the smallest molar mass.
Why do light gases effuse faster?
Their molecules move faster at the same temperature, so they escape through a small hole more quickly.
Related topics in Kinetic Theory & Statistical Physics
Law of atmospheres · Diffusion · Fick's law · Real gases · Van der Waals equation · Compressibility factor