Effusion Question

[NeuroscEYEnce]

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There might be a really obvious answer for this question, but I was watching Chad's Gen-Chem videos on Effusion and he compares the rate of H2 and O2. In the equation he puts O2 molecular mass divided by H2 molecular mass then the square-root and got the answer of 4 times faster. Which molecular mass do you put in the numerator and denominator in this Effusion equation?? Is it always the molecule with the higher mass?

 

[adrakdavra]

Mathematically, this can be represented as: Rate1 / Rate2 = square root of (Mass2 / Mass 1), so it is mass 2 on top, which is the heavier one,
A silly way to remember it , by thinking FAT gass and skinny gass, the Fat is bigger on top, but it is always slower , in effusion the fat gass ( larger) will be slower in rate. so Rate H / rate O = square root Mass O / Mass H = square root 16/1 = 4.

 

[NeuroscEYEnce]

Mathematically, this can be represented as: Rate1 / Rate2 = square root of (Mass2 / Mass 1), so it is mass 2 on top, which is the heavier one,
A silly way to remember it , by thinking FAT gass and skinny gass, the Fat is bigger on top, but it is always slower , in effusion the fat gass ( larger) will be slower in rate. so Rate H / rate O = square root Mass O / Mass H = square root 16/1 = 4.

Thank you!!!! 😀 😀 <3

 

[MD Odyssey]

A better way to think about it is to actually understand why effusion works the way that it does. Forget memorizing the equations.

Effusion occurs because, at a molecular level, a gas can be looked at as a bunch of molecules whizzing around with a certain amount of kinetic energy. The effusion rate is basically the number of particles per second that are passing through a very small hole. This necessitates the idea that faster the particles are moving, the higher the effusion rate. In other words, the effusion rate is proportional to the velocity.

If we assume both gases are at the same temperature, then the more massive the gas molecules, the lower its velocity. So, if you want to compare the effusion rate of two gases at the same temperature, the heavier gas molecules will, on average, be moving slower than the lighter ones. Therefore, the effusion rate for the lighter gas is going to be higher than the one for the heavier gas.

This also explains the presences of the square root sign. Kinetic energy is proportional to mass and also proportional to the square of the velocity. Since the effusion rate is proportional to the velocity, you can work backwards from there and see where the square root comes from.

The MCAT won't test whether you can regurgitate the formula. It will test whether you understand things like the reasoning that I outlined above.