Here the example question says calculate the ratio of the effusion rates of helium to methane. All right, so the first gas that's name represents our gas A and the second one will represent our gas B. So here we're going to say rate of helium divided by the rate of methane. We're not given times for them, so we can't use time in terms of this question. But we know the identity of the gases, therefore we know they're molar masses.

So this equals. Remember rate and molar mass are inversely proportional. So if the rate of helium is on the top, then the molar mass of helium has to be on the bottom. Because the rate of methane is on the bottom, the molar mass of methane has to be on top. Here we have one carbon and four hydrogens. When you add all of their masses together from the periodic table, we'll get its mass as 16.042g per mole for methane.

When you look up the atomic mass of helium on the periodic table, it's 4.003g per mole for helium. So divide 16.042 by 4.003 and take the square root and that gives us approximately 2.00187. So this would be the ratio in terms of the rate of effusions for helium to methane.

Now what is this number telling us? Well, what this number is telling us? It's telling us that helium moves twice as fast as methane, and this makes sense because helium weighs less. Remember, we said that the less you weigh, the faster you're going to be able to move as a gas. Because methane weighs more, it makes sense. It's not going to be able to move quite as fast as helium.

So again, when we're figuring out the rate of gas A to gas B, whatever is discussed first is gas A. Second one is gas B. The answer is telling us how much faster the top gas, in this case helium, is to methane.