will make up 20% of the total pressure. So if you’re told that the total pressure
within the container is 500 torr, you know that
• oxygen’s partial pressure is (0.20)(500) = 100 torr,• hydrogen’s partial pressure is (0.30)(500) = 150 torr,• nitrogen’s partial pressure is (0.50)(500) = 250 torr, and 100 + 150 + 250
= 500 torr.Partial Volumes
In the same way that you can use the molar ratio to determine the partial
pressure of a gas in a mixture, you can also determine the partial volume at
standard temperature and pressure (STP). Using the ideal gas equation, PV =
nRT, you can calculate that, at STP, 1 mole of a gas occupies a volume of 22.4 L.
But this value does not apply only to ideal gases—it’s also the accepted molar
volume for any gas at STP. This means that, at STP, the molar ratio in a gaseous
mixture will be directly proportional to the volume ratio, in a total volume of
22.4 L. For example, you can use the following formula to relate the molar ratio
of gas A in a mixture of many gases to its partial volume in that mixture:
# of moles of gas A = volume of gas A at STP# of total moles of gas = 22.4 LIf you know the ideal gas equation (PV = nRT) and what you’ve just learned
about partial pressures, you’ll be in good shape when the test asks you about
gases.
Now try these. Answers can be found in Part III.