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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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Gases: Mixtures and Movements >

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14.4 Dalton’s Law

Dalton’s Law

How is the total pressure of a mixture of gases related to 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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14.4 Gases: Mixtures and Movements > Dalton’s Law

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

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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SAMPLE PROBLEM

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14.6

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Practice Problems for Sample Problem 14.6

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Gases: Mixtures and Movements >

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14.4 Graham’s Law

Graham’s Law

How does the molar mass of a gas affect the rate at which the gas effuses or diffuses?

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Gases: Mixtures and Movements >14.4 Graham’s Law

Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.

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Gases: Mixtures and Movements >14.4 Graham’s Law

Bromine vapor is diffusing upward through the air in a graduated cylinder.

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Gases: Mixtures and Movements >14.4 Graham’s Law

After several hours, the bromine has diffused almost to the top of the cylinder.

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14.4 Gases: Mixtures and Movements > Graham’s Law

During effusion, a gas escapes through a tiny hole in its container.

Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

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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

Comparing Effusion Rates

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

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14.4 Gases: Mixtures and Movements > Graham’s Law

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

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.

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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.

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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