Graham's Law of Effusion
Lighter gases effuse faster.
About this topic
Graham's law states that the rate of effusion (or diffusion) of a gas is inversely proportional to the square root of its molar mass: rate1 / rate2 = √(M2 / M1). Lighter gases move faster at the same temperature, so hydrogen effuses about four times faster than oxygen.
Effusion is gas escaping through a tiny hole, while diffusion is gas spreading through another gas. At the same temperature, gases have the same average kinetic energy, 1/2 mv squared, so lighter particles must have higher speeds. The law lets you compare rates or times, and also work out an unknown molar mass. It is an approximation for diffusion, since collisions slow things down.
Worked example: compare helium (4.0 g/mol) with methane (16.0 g/mol). The ratio of rates is the square root of 16/4, which is 2, so helium effuses twice as fast. That is why helium balloons deflate faster than air-filled ones, as helium slips through the pores in the material. In history, the law was used during the Manhattan Project to separate uranium-235 from uranium-238 as uranium hexafluoride gas, a very small difference in molar mass.
A common error is inverting the ratio or forgetting the square root. A heavier gas is slower, so check that your answer makes sense.
Key ideas
- Rate ∝ 1/√M
- Hydrogen escapes fastest
- Used to enrich uranium
Quick check
Use cases: where Graham's Law of Effusion helps
- Homework: review the key idea behind Graham's Law of Effusion before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Graham's Law of Effusion without notes.
- Visual learning: play with the interactive lab to see how Graham's Law of Effusion changes when you adjust its inputs.
- Teaching a friend: explain Graham's Law of Effusion in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Graham's Law of Effusion?
Lighter gases effuse faster.
What will I learn in Graham's Law of Effusion?
Rate ∝ 1/√M. Hydrogen escapes fastest. Used to enrich uranium.
Is Graham's Law of Effusion part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Graham's Law of Effusion lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Graham's Law of Effusion?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.