1. Gases: Mixtures and Movements
Prentice-Hall Chapter 14.4
Dr. Yager

2. Objectives
Relate the total pressure of a mixture of gases to the partial pressures of the component gases
Explain how the molar mass of a gas affects the rate at which the gas diffuses and effuses

3. Dalton’s Law
In a mixture of gases, the total pressure is the sum of the partial pressures of the gases.

4. Partial Pressure
The contribution each gas in a mixture makes to the total pressure is called the partial pressure exerted by that gas.

5. Dalton’s Law of Partial Pressures
states that, at constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases.

6. Three gases are combined in Container T.= + +

7. The partial pressure of oxygen must be 10
The partial pressure of oxygen must be kPa or higher to support respiration in humans. The Mt. Everest climber below needs an oxygen mask and a cylinder of compressed oxygen to survive.

10. A gas mixture containing oxygen, nitrogen, and carbon dioxide has a total pressure of 32.9 kPa If PO2 = 6.6 kPa, and PN2 = 23.0 kPa, what is the pressure of the carbon dioxide?
Ptotal = sum of all the partial pressures
32.9 kPa = 6.6 kPa kPa + pressure of CO2
3.3 kPa = pressure of CO2

11. Diffusion vs Effusion
Diffusion is the tendency of molecules to move towards areas of lower concentration.
Effusion is the movement of a gas through a tiny hole in a container.

12. Key Idea:
Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

13. Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.
Bromine vapor is diffusing upward through the air in a graduated cylinder
After several hours, the bromine has diffused almost to the top of the cylinder.

14. Graham’s Law of Effusion
Graham’s Law states that the rate of effusion of a gas is inversely proportional to the square root of the gas’ molar mass.
This law can also be applied to the diffusion of gases.

15. A helium filled balloon will deflate sooner than an air-filled balloon.

16. Helium and air molecules pass freely through pores in the balloon
Helium and air molecules pass freely through pores in the balloon. Helium atoms are less massive than oxygen or nitrogen molecules.
At the same temperature (same kinetic energy):
KE = ½ mv m = mass, v = velocity
For constant KE, an increase in mass must be balanced by a decrease in velocity. So air molecules (oxygen, nitrogen) move more slowly than helium atoms with the same kinetic energy.

17. Because the rate of effusion is related only to a particle’s speed, Graham’s law can be written as follows for two gases, A and B.

18. Helium effuses (and diffuses) nearly three times faster than nitrogen at the same temperature.
Molar mass of N2 = 28 g
Molar mass of He = 4 g

19. 1. What is the partial pressure of oxygen in a diving tank containing oxygen and helium if the total pressure is 800 kPa and the partial pressure of helium is 600 kPa?
200 kPa
0.75 kPa
1.40 x 104 kPa
1.33 kPa

20. 1. What is the partial pressure of oxygen in a diving tank containing oxygen and helium if the total pressure is 800 kPa and the partial pressure of helium is 600 kPa?
200 kPa
0.75 kPa
1.40 x 104 kPa
1.33 kPa

21. 2. A mixture of three gases exerts a pressure of 448 kPa, and the gases are present in the mole ratio 1 : 2 : What are the individual gas pressures?
44 kPa, 88 kPa, and 316 kPa
52 kPa, 104 kPa, and 292 kPa
56 kPa, 112 kPa, and 280 kPa
84 kPa, 168 kPa, and 196 kPa

22. 2. A mixture of three gases exerts a pressure of 448 kPa, and the gases are present in the mole ratio 1 : 2 : What are the individual gas pressures?
44 kPa, 88 kPa, and 316 kPa
52 kPa, 104 kPa, and 292 kPa
56 kPa, 112 kPa, and 280 kPa
84 kPa, 168 kPa, and 196 kPa

23. 3. Choose the correct words for the spaces
3. Choose the correct words for the spaces. Graham's Law says that the rate of diffusion of a gas is __________ proportional to the square root of its _________ mass.
directly, atomic
inversely, atomic
inversely, molar
directly, molar

24. 3. Choose the correct words for the spaces
3. Choose the correct words for the spaces. Graham's Law says that the rate of diffusion of a gas is __________ proportional to the square root of its _________ mass.
directly, atomic
inversely, atomic
inversely, molar
directly, molar