Learn about Advanced Placement Chemistry, Gases 2, in this comprehensive video by bannanaiscool.
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So Graham saw something pretty cool on this formula. Take a look, he sees that you can determine the speed of the gas by the square root of 3RT over molar mass. But hey, if you have two different gases at the same temperature, the only difference in their speed is going to be at the same temperature how big they are in terms of their molar mass. So the larger the molar mass of a substance the slower it goes. The bigger something is at the same temperature the slower it goes. That makes sense. So Graham came up with a law of diffusion. This is what he said, it was quite fascinating. If you wanted to determine the rate of a gas compared to another gas let’s say the rate of gas 1 over the rate of gas 2 compare the two rates to each other then all you have to do is know the molar mass of gas two and divided by the square root of the molar mass of gas one. You'll be able to then determine that rate. Or if you know the rate you'd be able to determine molar masses. So if you have an unknown gas you can actually calculate its molar mass and then figure out its formula if you know the difference or what this rate difference is here and you know one of the molar masses of gases. So, let’s just look at this. If you have hydrogen gas and you want to compare it to the speed of hydrogen chloride gas what you'd want to do is take the molar mass of the HCl and divide it by the molar mass and it’s the square root of both of those of the H two. And this one is going to be faster than this because it’s a lighter gas, so we get the heavier molar mass and put it on top and that gives us the ratio here and 4.25 is actually the answer. This gas H2 at a given temperature will actually travel 4.25 times faster than the hydrogen chloride gas. That’s pretty cool.

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