© 2013 Pearson Education, Inc. Chapter 7, Section 9

General, Organic, and Biological Chemistry

Fourth EditionKaren Timberlake

7.9Partial Pressures

(Dalton’s Law)

Chapter 7Gases

© 2013 Pearson Education, Inc.Lectures

© 2013 Pearson Education, Inc. Chapter 7, Section 9

In a gas mixture, each gas exerts its partial pressure, which is the pressure it would exert if it were the only gas in the container.

Dalton’s law states that the total pressure of a gas mixture is the sum of the partial pressures of the gases in the mixture.

Partial Pressure, Dalton’s Law

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

Dalton’s Law of Partial Pressures indicates that pressure depends on the total number of gas particles.

Dalton’s Law of Partial Pressures

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

For example, at STP, one mole of a pure gas willexert the same pressure as one mole of a gasmixture in a 22.4 L container.

V = 22.4 L Gas mixtures

Total Pressure

0.5 mole O2

0.3 mole He0.2 mole Ar1.0 mole

1.0 mole N2

0.4 mole O2

0.6 mole He1.0 mole

1.0 atm 1.0 atm 1.0 atm

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Composition of Air

Air is a mixture of gases, including nitrogen, oxygen, carbon dioxide, argon, and water gases.

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Guide to Solving for Partial Pressure

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Learning Check

A scuba tank contains a mixture of He and O2 gases at 7.00 atm. If the pressure of O2 gas is 1140 mmHg, what is the partial pressure of He gas in the tank?

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

A scuba tank contains a mixture of He and O2 gases at7.00 atm. If the pressure of O2 gas is 1140 mmHg, whatis the partial pressure of He gas in the tank?

Step 1 Write the equation for partial pressures.

Step 2 Rearrange the equation to solve for the unknown pressure.

Solution

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

A scuba tank contains a mixture of He and O2 gases at7.00 atm. If the pressure of O2 gas is 1140 mmHg, whatis the partial pressure of He gas in the tank?

Step 3 Substitute known pressures into the equation and calculate the unknown partial pressure.

Solution

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

Blood Gases

In the lungs, O2 enters the blood, at the same time that CO2 is released.

In the tissues, O2 enters the cells, which release CO2 into the blood.

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© 2013 Pearson Education, Inc. Chapter 7, Section 9

Blood Gases

In the body, O2 flows into the tissues because the partial pressure of

O2 is higher in blood and lower in the tissues.

CO2 flows out of the tissues because the partial pressure of CO2 is higher in the tissues and lower in the blood.

Partial Pressures (mmHg) in Blood and TissueGas Oxygenated Deoxygenated Tissues

Blood Blood

O2 100 mmHg 40 mmHg 30 mmHg or less

CO2 40 mmHg 46 mmHg 50 mmHg or greater

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Changes in Partial Pressures of Blood Gases During Breathing

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