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Q & A: Measuring the Density of Gas

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Most recent answer: 12/11/2014
Q:
How do you measure the density of gas?
- Fiona (age 8)
Bronx, NY, USA
A:
Fiona -

There are two good ways of doing this. The first is that you can take a certain amount of gas (in a balloon, for example) and weigh it on a scale (if it's heavier than air) or measure how high it floats (how 'bouyant' it is - if it's lighter than air). Then you can compare how heavy/bouyant it is to the density of air to figure out how dense the gas is.

The other way uses more math. The density of a gas is equal to the gas's pressure divided by a constant number (R=0.08206 L*atm/K) and the temperature (in degress Kelvin). (Or d = n/V = P/RT) So if you know the temperature and the gas's pressure (the pressure of air is 1 atm), you can figure out it's density.

-Tamara

(published on 10/22/2007)

Follow-Up #1: density of gas

Q:
I think you're wrong on this answer. The gas law PV = nRT is true for any gas of any density. You say that d = n/V = P/RT. That's true, but n = number of moles, not mass, so n/V is not density.
- Dave Hermeyer (age 66)
San Francisco
A:

Thanks for your helpful comment, which makes an important point. Before going further, it's important to note that the ideal gas law pV=nRT is not "true for any gas of any density." It's an approximation that works well for ideal gases, i.e. ones at low enough density.

On your main point, it's true that using that law gives the number density not the mass density described in the other parts. To go from one to the other would require somehow knowing the molecular weight of the gas. If the original question was about making practical measurements now about some known type of gas, you can of course just look up the molecular weight. If it was about how people could have done the whole thing from scratch or if it was about a gas of unknown composition, then you couldn't just look up the molecular weight. So in those cases, you should scratch that ideal gas law method for finding the mass density and stick to the others. If what's wanted is the number density, you should scratch the other methods and stick to the (approximate) ideal gas law method.

Mike W.

p.s. Some other answers here are more complete:



 

 



 


(published on 12/11/2014)

Follow-up on this answer.