1. Partial Pressure-Intro
Consider these boxes, all the same V and T:
 If P is 3 atm in the H2 box, what is P in the Ar box?
 If P is 3 atm in the H2 box, what is P in the Ar box? (Hint: there are twice as many particles)
 And what would be the pressure if the two gases were put into the same box (at same V and T)? Ar
PAr = _____
6 atm
Mixture of Ar and H2 H2
PH2 = 3 atm
Ptotal = _____
9 atm

2. Partial Pressures (in Gas Mixtures)
The partial pressure of a gas A (PA) is the pressure that Gas A would exert if it were the only gas in the vessel.
Consider a box with volume Vbox with two gases in it, A and B, and at given temperature, T.
ntot =
nA + nB
(Dalton’s Law)
Ptot = PA + PB
Chapter 09
Slide 2

3. Back to Earlier Example
Mixture of Ar and H2
Ptotal = PH2 + PAr = 3 atm + 6 atm = 9 atm
Chapter 09
Slide 3

4. Copyright © Houghton Mifflin Company. All rights reserved.
Figure 5.12, Zumdahl
The Partial Pressure of each Gas in a Mixture of Gases in a Container Depends on the Number of Moles of that Gas
Copyright © Houghton Mifflin Company. All rights reserved.

5. Dalton’s Law of Partial Pressures 05
For a two-component system, the moles of components A and B can be represented by the mole fractions (XA and XB).
NOTICE: XA + XB = 1
Chapter 09
Slide 5

6. Dalton’s Law of Partial Pressures 06
Mole fraction is related to the total pressure by:
On a humid day in summer, the mole fraction of gaseous H2O (water vapor) in the air at 25°C can be as high as Assuming a total pressure of atm, what is the partial pressure (in atm) of H2O in the air?
Chapter 09
Slide 6

7. Dalton’s Law of Partial Pressures 02
2.0 moles of Ne and 3.0 moles of Ar were placed in a 40.0 L container at 25oC. What are the partial pressures of each gas and the total pressure?
Chapter 09
Slide 7

8. Dalton’s Law of Partial Pressures 02
A sample of natural gas contains 6.25 moles of methane (CH4), moles of ethane (C2H6), and moles of propane (C3H8). If the total pressure of the gas is 1.50 atm, what are the partial pressures of the gases?
Chapter 09
Slide 8

9. Dalton’s Law of Partial Pressures 03
Hydrogen gas generated when calcium metal reacts with water is collected at 30°C and a pressure of 988 mm Hg. The volume collected is 641 ml. What is the mass (in grams) of the hydrogen gas obtained? The pressure of the water vapor at 30°C is 32 mm Hg.
760 mm Hg = 1.00 atm
Chapter 09
Slide 9

10. Graham’s Law
Diffusion is the mixing of different gases by random molecular motion and collision.
Effusion is when gas molecules escape without collision, through a tiny hole into a vacuum.
Chapter 09

11. Graham’s Law 03 For two gases at same temperature and pressure:
(Again, “Big guys move more slowly at the same T!”)
What are the relative rates of diffusion of the three naturally occurring isotopes of neon: 20Ne, 21Ne, and 22Ne?
Chapter 09