What is Graham's Law?: Definition with Example

    Grahams Law: Graham's law states that the rate of diffusion or of effusion of a gas is inversely proportional to the square root of its molecular weight. Thus, if the molecular weight of one gas is four times that of another, it would diffuse through a porous plug or escape through a small pinhole in a vessel at half the rate of the other (heavier gases diffuse more slowly). A complete theoretical explanation of Graham's law was provided years later by the kinetic theory of gases.

           We can separate the isotopes of an element using Graham's law. A common example is enriching uranium from its isotope. U is desired uranium which amounts 0.72 % of natural uranium. The separation of 235U from the rest isotope 238U is achieved by passing uranium hexafluoride gas into porous membranes.

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