Which of the following gases has the lowest rate of diffusion under the same condition? [H = 1.00; He = 4. 00; O = 16.0; CI = 35.5]

The gas with the lowest rate of diffusion under the given conditions is the gas that is the heaviest, which means it has the highest molar mass. The molar masses of the gases listed in the question are: H₂ (2.0 g/mol), He (4.0 g/mol), O₂ (32.0 g/mol), and CI₂ (71.0 g/mol). Therefore, the gas with the lowest rate of diffusion is CI₂, because it has the highest molar mass. This means that CI₂ molecules are larger and heavier than molecules of the other gases listed, and so they move more slowly and take longer to diffuse through a medium. In summary, the heavier the gas, the slower it diffuses, and the lighter the gas, the faster it diffuses.