where r 1 and M 1 represent the diffusion/effusion rate and molar mass of gas 1, and r 2 and M 2
represent the diffusion/effusion rate and molar mass of gas 2. In general, this means that larger gas
molecules diffuse/effuse at a slower rate than smaller gas molecules.
A CLOSER LOOK
One common application of Graham’s law is in the separation of isotopes; for example, in
the enrichment of uranium for nuclear reactors. Most of the naturally occurring atoms of
uranium are U-238, while a few are the U-235 isotopes used as fuel for nuclear fission
reactors. This lighter isotope is collected and concentrated by letting a sample of uranium
(compounded with fluorine) effuse into an evacuated container. The fluoride compound
with the lighter isotope would diffuse faster and thus be present in a higher concentration in
the second container. This procedure is repeated several times to attain the desired isotopic
purity.