1 Chapter 11 Gases 11.10 Partial Pressure (Dalton’s Law) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings.

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Chapter 11 Gases

11.10

Partial Pressure (Dalton’s Law)

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The partial pressure of a gas• Is the pressure of each gas in a mixture.• Is the pressure that gas would exert if it were

by itself in the container.

Partial Pressure

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Dalton’s Law of Partial Pressures statesthat the total pressure• Depends on the total number of gas

particles, not on the types of particles.• Exerted by a gas mixture is the sum of

the partial pressures of those gases.•

PT = P1 + P2 + .....

Dalton’s Law of Partial Pressures

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Illustrating Partial Pressures

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At STP, • One mole of a pure gas in a volume of 22.4 L will

exert the same pressure as one mole of a gas mixture in 22.4 L.

• VSTP = 22.4 L Gas mixtures

Total Pressure

0.5 mol O2

0.3 mol He0.2 mol Ar1.0 mol

1.0 mol N2

0.4 mol O2

0.6 mol He1.0 mol

1.0 atm 1.0 atm 1.0 atm

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Scuba Diving

• When a scuba diver makes a deep dive, the increased pressure causes N2(g) to dissolve in the blood.

• If the rise is too fast, the dissolved N2 forms bubbles in the blood, a dangerous condition called "the bends".

• Helium is mixed with O2 to prepare breathing mixtures for deep descents.

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Learning Check

A scuba tank contains O2

with a pressure of 0.450 atm and He at 855 mm Hg.

What is the total pressure in mm Hg in the tank?

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1. Convert the pressure in atm to mm Hg 0.450 atm x 760 mm Hg = 342 mm Hg = PO2

1 atm

2. Calculate the sum of the partial pressures.

Ptotal = PO2 + PHe

Ptotal = 342 mm Hg + 855 mm Hg

= 1197 mm Hg

Solution

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For a deep dive, some scuba divers are using a mixture of helium and oxygen gases with a pressure of 8.00 atm.

If the oxygen has a partial pressure of 1280 mm Hg, what is the partial pressure of the helium?

1) 520 mm Hg

2) 2040 mm Hg

3) 4800 mm Hg

Learning Check

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3) 4800 mm Hg PTotal = 8.00 atm x 760 mm Hg = 6080 mm Hg

1 atm PTotal = PO + PHe 2

PHe = PTotal - PO2

PHe = 6080 mm Hg - 1280 mm Hg = 4800 mm Hg

Solution

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Gases Collected Over Water

A gas produced in thelaboratory • Usually contains

water vapor.• PT = P water + P gas

• Has a partial pressure obtained by subtracting the vapor pressure of water from the PT.

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Learning Check

The decomposition of KClO3 produces O2 gas and solid KCl. If 124 mL of O2 is collected over water at 762 mm Hg and 20.0C, how many moles of O2 were produced?

2KClO3(s) 2KCl(s) + 3O2(g)

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Solution

Partial pressure water at 20.0C = 18 mm Hg

PT = P water + P gas

762 mm Hg = 18 mm Hg + PO2

PO2 = 762 mm Hg - 18 mm Hg = 744 mm Hg

PV = nRT

n = PV = (744 mm Hg)(0.124 L) = 0.00505 mol O2

RT (62.4 L • mm Hg)(293 K) mol • K

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Gases We Breathe

The air we breathe • Is a gas mixture.• Contains mostly

N2 and O2 and small amounts of other gases.

Table 11.7

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Blood Gases

• In the lungs, O2 enters the blood, while CO2 from the blood is released.

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

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Blood Gases

In the body, • Cells use up O2 and give off CO2.

• O2 flows into the tissues because the partial pressure of O2 is higher (100 mm Hg) in oxygenated blood, and lower (<30 mm Hg) in the tissues.

• CO2 flows out of the tissues because the partial pressure of CO2 is higher (>50 mm Hg) in the tissues, and lower (40 mm Hg) in the blood.

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Gas Exchange During Breathing

Table 11.8

Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings