GRAHAM'S LAW
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Graham's law, known as Graham's law of effusion, was formulated by Scottish physical chemist Thomas Graham in 1846. Graham found experimentally that the rate of effusionof a gas is inversely proportional to the square root of the mass of its particles. This formula can be written as:
Rate 1 = √ M2 Rate 2 M1 where: Rate1 is the rate of effusion of the first gas (volume or number of moles per unit time). Rate2 is the rate of effusion for the second gas. M1 is the molar mass of gas 1 M2 is the molar mass of gas 2. Graham's law states that the rate 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. A complete theoretical explanation of Graham's law was provided years later by the kinetic theory of gases. Graham's law provides a basis for separating isotopes by diffusion — a method that came to play a crucial role in the development of the atomic bomb. Graham's law is most accurate for molecular effusion which involves the movement of one gas at a time through a hole. It is only approximate for diffusion of one gas in another or in air, as these processes involve the movement of more than one gas. |
Check out this video and answer the problems below.1. What is the rate of effusion for a gas that has a molar mass twice that of a gas that effuses at a rate of 3.62 mol/min?
2. Calculate the rate of effusion of NO2 compared to SO2 at the same temperature and pressure. 3. Assume you have a sample of hydrogen gas containing H2, HD, and D2 that you want to separate into pure components. What are the various ratios of relative rates of effusion? |