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Q & A: Measuring Gas Density

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Q:
How do you measure the density of gas?
- Anonymous
A:
There are several ways to measure the density of gas. One way is to take a certain amount of gas and weigh it. (Yes, you really can weigh a gas. One way to do this is by filling a container with the gas and weighing it, then using a vacuum pump to empty the container out completely and weighing it again.) If you know how much the gas weighs and how much space it takes up, you can figure out the density. (Density = mass / volume)

Another way to do this is by taking advantage of something called the 'Ideal Gas Law' or PV=nRT. In this equation, (which is close to true for ordinary gases) P is pressure, V is volume, n is the number of moles (a mole is 6.022*10^23 molecules) of gas, R is a constant that other people have measured and T is the absolute temperature. To use this equation, all you have to do is measure the pressure of the gas and measure the temperature. From this, you can use the equation to find n/V, (n/V=P/RT). Measuring the pressure of a gas is fairly easy, and there are lots of instruments made to do just that.If you know what type of gas you're measuring, you can look up its molecular weight, which tells you how to convert from moles per volume to grams per volume. Of course, the table of molecular weights was made by actually weighing the gases.


-Tamara & Mike

(republished on 07/17/06)

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