Graham’s law

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Graham’s law, named after the Scottish chemist Thomas Graham, describes the relationship between the rates of diffusion or effusion of two gases and their molar masses. The law states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass. For example, consider the diffusion of helium (with a molar mass of about 4 g/mol) and sulfur hexafluoride (with a molar mass of about 146 g/mol). According to Graham’s law, helium, being lighter, will diffuse much faster than sulfur hexafluoride under the same conditions. This principle highlights the fundamental idea that lighter gases diffuse or effuse more rapidly than heavier gases.

This relationship is mathematically expressed as follows:

$$\frac{R_{1}}{R_{2}} = \sqrt{\frac{M_{2}}{M_{1}}}$$

Where R1 and R2 represent the rates of diffusion or effusion of two different gases, and M1 and M2 represent their respective molar masses. This equation highlights the fundamental principle that the rate at which a gas can move through a medium is influenced by its molar mass.


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