# Calculating Gas Volume at STP

I had a question on a test which asked me the volume of sulphur dioxide that would be formed if 12.7g of copper were used. How do you calculate gas volume when you only have the mass?

At STP, 1 mol of any gas is equal to 22.4L. So if you know the moles of sulfur dioxide you can multiply that by 22.4 to get the volume.

Borek
Mentor
That's assuming you are asked for volume at STP and assuming you know what STP means. STP has no one, accepted definition and T/p values used depend on the application, defining body and so on. See wikipedia article on STP for details.

The most general approach is to use ideal gas equation, pV=nRT. Using 22.4L is only a shortcut, one that can give dangerously wrong answer.

Thanks a lot Renge. I knew it was a constant I just couldn't find the constant on google. I best memorize that number.

Borek this is real basic chemistry we're doing at the moment the teacher wrote that constant (22.4L) on the board I just forgot it. It was STP I forgot to mention that. I'm gonna learn that ideal gas equation though because chemistry is the field I'm planning on getting into. Thanks.

Borek
Mentor
There is nothing wrong with using some shortcuts and simplifications at some point when learning an idea, it is just good to know beforehand that it is just a simplification.

I have a question related to this thread. So if the STP of any gas is 22.4L per mole, all you have to do is to multiply 22.4L with the number of moles of the specified gas? High-school level chemistry is what I'm doing, so as of right now I need not learn that equation...

Borek
Mentor
So if the STP of any gas is 22.4L per mole

VOLUME of 1 mole of gas at STP is 22.4L.

all you have to do is to multiply 22.4L with the number of moles of the specified gas?

If you are not given exact conditions (T,P) that's often the best approximation.