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Chemistry 14.4
Ideal Gases and Real Gases
Under what conditions are real gases
most likely to differ from ideal gases?
Ideal gases obey the gas laws and behave according to the Kinetic Molecular Theory. No gas is a true ideal gas but He and H2 behave most like an ideal gas especially when P is low and T is high.
There are attractions between the particles in a gas. Because of these attractions, a gas can condense,or even solidify, when it is compressed or cooled.
Real gases differ most from an ideal gas at low temperatures and high pressures.
Gases behave most like an ideal gas at high temperatures and low pressures.
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14.4 Gases: Mixtures and Movements
A list of gear for an expedition to Mount Everest includes climbing equipment, ski goggles, a down parka with a hood, and most importantly compressed-gas cylinders of oxygen. You will find out why a supply of oxygen is essential at higher altitudes.
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Dalton’s Law
How is the total pressure of a mixture of gases related to the partial pressures of the component gases?
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.
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Gases: Mixtures and Movements >14.4 Dalton’s Law
The contribution each gas in a mixture makes to the total pressure is called the partial pressure exerted by that gas.
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Gases: Mixtures and Movements >14.4 Dalton’s Law
Three gases are combined in container T.
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Gases: Mixtures and Movements >14.4 Dalton’s Law
The partial pressure of oxygen must be 10.67 kPa or higher to support respiration in humans. The climber below needs an oxygen mask and a cylinder of compressed oxygen to survive.
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SAMPLE PROBLEM
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14.6
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SAMPLE PROBLEM
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14.6
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SAMPLE PROBLEM
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14.6
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Practice Problems for Sample Problem 14.6
INDEX CARD activity (turn in for evaluation)
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Graham’s Law
How does the molar mass of a gas affect the rate at which the gas effuses or diffuses?
Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.
During effusion, a gas escapes through a tiny hole in its container.
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14.4 Gases: Mixtures and Movements > Graham’s Law
Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.
Thomas Graham’s Contribution: Graham’s law of effusion states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass. This law can also be applied to the diffusion of gases.
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14.4 Gases: Mixtures and Movements > Graham’s Law
Comparing Effusion Rates
A helium filled balloon will deflate sooner than an air-filled balloon.
Helium atoms are less massive than oxygen or nitrogen molecules. So the molecules in air move more slowly than helium atoms with the same kinetic energy.
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14.4 Gases: Mixtures and Movements > Graham’s Law
Thomas Graham’s Contribution
Graham’s law of effusion states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass. This law can also be applied to the diffusion of gases.
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14.4 Gases: Mixtures and Movements > Graham’s Law
Helium effuses (and diffuses) nearly three times faster than nitrogen at the same temperature.
Animations and practice problems for diffusion and effusion
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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
14.4 Section Quiz.
14.4.
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14.4 Section Quiz.
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?
a. 200 kPa
b. 0.75 kPa
c. 1.40 104 kPa
d. 1.33 kPa
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14.4 Section Quiz.
2. A mixture of three gases exerts a pressure of 448 kPa, and the gases are present in the mole ratio 1 : 2 : 5. What are the individual gas pressures?
a. 44 kPa, 88 kPa, and 316 kPa
b. 52 kPa, 104 kPa, and 292 kPa
c. 56 kPa, 112 kPa, and 280 kPa
d. 84 kPa, 168 kPa, and 196 kPa
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14.4 Section Quiz.
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.
a. directly, atomic
b. inversely, atomic
c. inversely, molar
d. directly, molar
