Volume of Two Gases

Staff Emeritus
I had an exam question where I had to find the volume taken up by a gas consisting of 32 grams of oxygen and 4 grams of helium at STP conditions. I ended up getting a supposedly incorrect answer of about 44.8 liters by finding the volume of both gases and adding them together. However, my instructor said that it's actually half that, 22.4 liters, and that you don't add the volume of two gases together in this situation. When I asked why, he couldn't really explain it in any way that made sense to me and we got into a confusing discussion over when the ideal gas law applies and when it doesn't. The whole thing ended up confusing me.

So, in this situation do you add the volume taken up by the two gases together or not?

Borek
Mentor
The only thing that matters is number of moles. 4 gram of helium is 1 moles, 32 grams of oxygen is another mole. All in all there are two moles of gas, and the volume is that of two moles - so 44.8 L at STP.

Unless teh question was worded differently, 44.8 L is a correct answer.

Chestermiller
Bystander
Homework Helper
Gold Member
So, in this situation do you add the volume taken up by the two gases together or not?
Sadly, people get PhDs, teaching positions, and other jobs and recognition as "scientists/experts" without ever understanding the difference between mole fraction and partial pressure, and when to use each.

Staff Emeritus
I remember part of his reasoning was that he thought that two different gases don't interact with each other under the assumptions of an ideal gas. Even assuming this, wouldn't the pressure on the container walls still be the same as when they are interacting with each other? If one gas exerts X pressure on the walls of a container, and another gas exerts Y pressure on the walls of a container, then the pressure when you combine them would seem to be X+Y even if the two gases passed right through each other.

Bystander
Homework Helper
Gold Member
he thought that two different gases don't interact with each other under the assumptions of an ideal gas.
This misunderstanding of the properties of "ideal gases" is as good a root cause for "partial pressure - mole fraction" confusion as any; "point masses" imply no intermolecular collisions, however, it has never been necessary to exclude collisions from discussion of ideal gas properties, one need only constrain collisions to being perfectly elastic. Never get any good chemical kinetics from the model, but that's not the point of a thermodynamic model.

Staff Emeritus
Thanks, guys.

On a related note, what does "cavitation" mean in the following context:

The ideal gas law is often introduced in its common form:

where P is the cavitation pressure of the gas, V is the cavitation volume of the gas, n is the cavitation amount of substance of gas (measured in moles), R is the ideal, or universal, cavitation gas constant, and T is the cavitation temperature of the gas.

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Bystander
Homework Helper
Gold Member
"cavitation"
This is from some larger discussion regarding bubble formation in liquids? Never heard the word being used in any other context, and wouldn't think it's any sort of general discussion of boiling if it's using an ideal gas model.

Bystander