Measuring the Density of Gas
Most recent answer: 10/22/2007
- Fiona (age 8)
Bronx, NY, USA
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 its heavier than air) or measure how high it floats (how bouyant it is - if its 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 gass 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 gass pressure (the pressure of air is 1 atm), you can figure out its density.
-Tamara
(published on 10/22/2007)
Follow-Up #1: density of gas
- Dave Hermeyer (age 66)
San Francisco
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)