Imagine you have two gases, X and Y. Gas X has a molar mass of 208 g/mol and Gas Y is 82 g/mol. Which gas is predicted to be the faster of the two? By how much?

Answer 1

Refer to Graham's Law of Effusion, which essentially states that the rate of effusion is inversely proportional to the molar mass of the gas. The heavier the gas, the slower it will move. Thus, we predict that gas Y will move faster. To determine by how much, we use Graham's law:

rate gas 1 / rate gas 2 = sqrt MW gas 2 / sqrt MW gas 1

rate X / rate Y = sqrt 82 / sqrt 208 = 9.06 / 14.4

or put another way, Y will be about 1.6 time faster than X (14.4 / 9.06)