In chemistry, effusion is the process where individual molecules flow through a hole without collisions between molecules. This occurs if the diameter of the hole is considerably smaller than the mean free path of the molecules. According to Graham's law, the rate at which gases effuse (i.e., how many molecules pass through the hole per second) is dependent on their molecular weight; gases with a lower molecular weight effuse more quickly than gases with a higher molecular weight. For two gases at the same temperature (and having the same specific heat), and thus having the same kinetic energy, the average molecular speed of each gas can be found using the equation E=(1/2) mv^2. (To be more accurate, one would use the equation (3/2)kBT = (1/2)mvrms2 where kB is the Boltzmann constant and vrms is the root mean square speed of the gas molecules. The average speed would then be about 0.921 vrms.) Thus, lighter molecules have a higher speed. This results in more molecules passing through the hole per unit time. This is why a balloon filled with low molecular weight hydrogen left alone for a reasonable amount of time deflates faster than a separate balloon full of higher molecular weight oxygen.

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