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Formula For Grahams Law of Diffusion

Chemistry Formulas

Graham’s Law of Diffusion :

About Graham’s Law of Diffusion

A gas expands to fill the entire container even if other gas(es) is (are) already present in the container. This process of spreading of gas is called as diffusion. A gas confined to a container at high pressure than the surrounding atmosphere will escape from a small hole which is opened in the container until the pressure outside and inside have been equalized. This process is called as effusion. For More Chemistry Formulas just check out main page of Physics Wallah.

Example of effusion: Escaping of air through a punctured tyre.

Schottish physical chemist, Thomas Graham’s research on the diffusion of gases was triggered by his reading about the observation of Germen chemist Johan Dobereiner that Hydrogen gas effused out of a small crack in a glass bottle faster than the surrounding air diffused in to replace it. Graham measured the rate of effusion of different gases experimentally through very fine tube. In this way he slowed down the process, so that it could be studied quantitatively. 

He plotted the following graph for complete effusion of 100 ml of different gases at 25oC.

grahams law of diffusion

It has been found that the rate of diffusion (r) is also proportional to the pressure of a gas (or number of molecules) at a given temperature. In that case, the rate of diffusion is given as :

grahams law

Solved Example of Graham's Law

rate of effusion of ethane

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