Chemistry: Graham’s Law

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Graham’s Law


Diffusion is a simple process that can be explained by kinetic theory. When you open a bottle of perfume, it can very quickly be smelled on the other side of the room. This is because as the scent particles drift out of the bottle, gas molecules in the air collide with the particles and gradually distribute them throughout the air. Diffusion of a gas is the process where particles of one gas are spread throughout another gas by molecular motion.


Figure 3. Diffusion of gas molecules into a less populated region.*

But not all gases diffuse at the same rate. Figure 3 above shows the perfume as being composed of all the same molecules. But in reality the perfume would be composed of many different types of molecules: some larger, more dense molecules and other smaller, lighter molecules. Thomas Graham, a Scottish chemist, discovered that lightweight gases diffuse at a much faster rate than heavy gases. Graham’s law of diffusion shows the relationship between diffusion and molar mass.

rate of diffusion1molar mass−−−−−−−−−√(1.9)(1.9)rate of diffusion∝1molar mass

Consider hydrogen and oxygen. The molar mass of hydrogen is about 1.0 grams/mol and the molar mass of oxygen is about 16 grams/mol. If you insert these values into the equation above, you get that the rate of diffusion for hydrogen is 1 and the rate of diffusion for oxygen is 1/4. This means that hydrogen will diffuse four times faster than oxygen.

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