Since gases are mostly empty space, the densities of gases are reported in g/L,
notg/mL as found for solids and liquids. As you’re probably aware, densityis equal to mass per unit of volume. To calculate the density of a gasat standard temperature and pressure, you take the molecular formulaweight of the gas (grams per mole—from the periodic table) and dividethat by the
standard molar volume for a gas, which is
22.4 L per mole:
where the formula weight (
FW) is in g/mol, and the standard molar volume is 22.4 L/mol. Now try using this in a problem.
Example
What is the density of helium gas at STP?
Explanation
If the density of the gas is equal to
, then
d = 4.00 g/mol ∏ 22.4 L/mol, so the density = 0.179 g/L.
If conditions are not standard, we can use this expanded version of the ideal gas equation:
Another really handy rearrangement of the ideal gas equation can be used to find the molecular weight of an unknown gas
.You’ll get a chance to practice using these in the problems at the endof the chapter. However, there is no need to memorize these lastequations since they are all rearrangements of the ideal gas law. Okay,two more important laws and then we’re finished with our discussion ofgases, and we move on to solutions.
