Density of Gases
Since gases are mostly empty space, the densities of gases
are reported in g/L, not g/mL as found for solids
and liquids. As you’re probably aware, density is equal to mass
per unit of volume. To calculate the density of a gas at standard
temperature and pressure, you take the molecular formula weight
of the gas (grams per mole—from the periodic table) and divide that
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.
What is the density of helium gas at STP?
If the density of the gas is equal to
, then d
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 end of the chapter. However,
there is no need to memorize these last equations since they are
all rearrangements of the ideal gas law. Okay, two more important
laws and then we’re finished with our discussion of gases, and we
move on to solutions.