Gases - Chemistry - page 255

We will also need to know the pressure exerted by the water vapor at
25 o C, which, according to the Vapor Pressure of Water table, is 23.8 mm
of Hg. Now, we are ready to solve the problem.
P dry hydrogen = P total - P H2O
P dry hydrogen = 743.3 mm of Hg - 23.8 mm of Hg = 719.5 mm of Hg

The only challenge to this variation on Dalton's Law is that it is un-
likely that anyone will remind you to use a table such as the one presented
in this lesson (Vapor Pressure of Water). Just try to associate questions
about water vapor pressure to this type of table. If a question talks about a
gas “collected over water” or if it asks for the “pressure of the dry gas,”
then you probably will need a table of water vapor pressures.

Lesson 8-4 Review

1. A mixture of helium and hydrogen gases exerts a combined pressure

of 2.5 atm. If the partial pressure of the hydrogen gas is 0.8 atm, what
is the partial pressure exerted by the helium gas?

2. A mixture contains argon, neon, and hydrogen gases. If the partial

pressures of these gases are 45.5 kPa, 23.6 kPa, and 85.3 kPa
respectively, what is the total pressure exerted by the mixture?

3. A sample of hydrogen gas is generated and collected by displacing

water at 30.0 o C. If the pressure of the resultant hydrogen and water
vapor mixture is 745.3 mm of Hg, what is the pressure of the dry
hydrogen gas?

4. A flask containing 2.0 moles of oxygen and 2.0 moles of carbon

dioxide has a total pressure of 6.0 atm. What is the partial pressure of
the oxygen gas?

5. What is the total pressure of a gaseous mixture containing helium,

oxygen, and carbon dioxide, if the partial pressure exerted by each
gas is 25.0 kPa?

6. A vessel contains a mixture of neon and argon with a total pressure

of 1.8 atm. If the partial pressure of the neon gas is 0.5 atm, what is
the pressure of the argon gas?
a) 0.5 atm b) 1.3 atm c) 1.8 atm d) 2.3 atm