I know this is probably some basic math once you have the figures, but my Google-fu is failing me.
Say I have 5 pounds of air. Air meaning, the typical composition of the atmosphere of Earth. And by 5 pounds I mean the weight of the mass of the air, not it's pressure or anything like that.
I want to know approximately how much volume that amount of air would take up at a given pressure — specifically, sea level pressure. I know that can vary — lets just say its a typical warm summer day, or just an average. I'm not looking for exact numbers.
Actually, I'd rather a subjective interpretation of the exact figure if you can offer one. Something like 'about the volume of the air in a 12×12 room', or what have you. I have no idea how big 5 pounds of air is.
Best Answer
The easy way to answer this is to just look up the density of air $\rho$ under whatever conditions you need it and then calculate $V=m/\rho$.
If you don't have a ready source for the density you need, though, you can use the ideal gas law,
$$PV=nRT$$
Here $n$ is the number of moles of the gas, which you can get by dividing the mass by the average molar mass of the gas:
$$n=\frac{m}{\mu}$$
The average molar mass of air would be a weighted average of the molar masses of its constituent gases. This is something you look up for the composition of air under your specified conditions, but it'll be around 29 g/mol. This at least does not depend directly on temperature.
If you need more accuracy, you could use something like the Van der Waals equation.