Example 1
Find the molar mass of a gas when the density is given as 1.25 grams/liter.
Because it is known that 1 mole of a gas occupies 22.4 liters at STP, we can
solve this problem by multiplying the mass of 1 liter by 22.4 liters/mole using
dimensional analysis:
Even if the mass given is not for 1 liter, the same setup can be used.Example 2
If 3.00 liters of a gas weighs 2.00 grams, find the molar mass.
Use dimensional analysis:
You can also find the density of a gas if you know the molar mass. Since the
molar mass occupies 22.4 liters at STP, dividing the molar mass by 22.4 liters
will give you the mass per liter, or the density.
Example 3
Find the density of oxygen at STP.
Oxygen exists naturally as a diatomic molecule. The molar mass of O 2 is
32.0 g/mol.
Use dimensional analysis:
Example 3 shows that you can find the density of any gas at STP by dividing its
molar mass by 22.4 L.
TIP
For a dry gas at STP:
density