Graham’s law of diffusion

Aug 26, 2022, 16:45 IST

About graham’s law of diffusion

If a tiny hole is made in the wall of a vessel containing a gas, then the rate at which gas molecules leak out of vessel will be proportional to the number of molecules that collide with unit area of the wall in unit time.

Diffusion is the tendency of any substance to spread throughout the space available to it. Diffusion will take place in all direction and even against gravity. So gases diffuse through firm substances and through small holes. The streaming of gas molecules through a small hole is called effusion

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.

Instantaneous rate of diffusion

Instantaneous rate of effusion will depend on the instantaneous partial pressure of that      component, which decreases continuously with progress of effusion. Considering, instantaneous rate of decrease of partial pressure (−dp/dt) to be directly proportional to instantaneous gas pressure and inversely proportional to square root of molar mass, instantaneous pressure at any time

Formulas of graham’s law of diffusion