what is Graham's law of diffusion?



Answers:
The rate at which gases diffuse is inversely proportional to the square root of their densities.

Since volumes of different gases contain the same number of particles (see Avogadro's Hypothesis), the number of moles per liter at a given T and P is constant. Therefore, the density of a gas is directly proportional to its molar mass (MM).
Diffusion — the tendency of molecules and ions to move toward areas of lower concentration until the concentration is uniform throughout the system.

Scotsman Thomas Graham did work in the 1840’s on effusion — the rate at which a gas escapes through a tiny hole in a container.

He noticed that gases with small molecular masses effuse faster than those with a high molecular mass. From his observation, he developed Graham’s law:

The rate of effusion of a gas is inversely proportional to the square root of its mass.

This relationship was later found to be true for diffusion as well.

This relationship is can be understood when considering mass and speed of a moving body and the force the body exerts when it strikes a stationary object.

We know that Kinetic energy = 1/2 mv2

If two bodies of unequal mass have the same kinetic energy, which moves faster?

The lighter one!

Thus, for two gases at the same temperature, the one with lower molecular mass will diffuse faster.

Graham’s Law can be written

where A and B are two gases.
diffusion rate of a gas/difussion rate of gas


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