Chemistry: Need a walk through on this problem?

Question

If 4.83mL of an unknown gas effuses through a hole in the same time it takes 9.23mL of Ar to effuse through the same hole, under the same conditions, what is the molecular weight of the unknown gas?

ANy help appreciated. My text book isn't very helpful. It gives one formula but no explanation and I cant use the formula for this problem.

Thanks

Answer 1

Graham's Law applies here. It says that the rate is inversely proportional to the sq. root of the molecular mass ratios. This simply means that a heavier gas moves slower (by the inverse of the sq. root, etc.)

It seems like you unknown gas is moving slower than argon at the ratio of 4.83 / 9.23

Invert this and you have the ratio of the sq. root of their respective masses. Look up the mass of argon and plug in to solve for the unknown.