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Chapter 11 Gases

11.7 Volume and Moles (Avogadro’s Law)

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

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Avogadro's Law: Volume and Moles

In Avogadro’s Law

• The volume of a gas is directly related to the number of moles of gas.

• T and P are constant.

V1 = V2

n1 n2

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

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Learning Check

If 0.75 mol of helium gas occupies a volume of 1.5 L, what volume will 1.2 mol of helium occupy at the same temperature and pressure?

1) 0.94 L

2) 1.8 L

3) 2.4 L Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Solution

3) 2.4 LConditions 1 Conditions 2V1 = 1.5 L V2 = ???n1 = 0.75 mol He n2 = 1.2 mol HeV2 = V1n2

n1

V2 = 1.5 L x 1.2 mol He = 2.4 L 0.75 mol He

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• The volumes of gases can be compared when they have the same conditions of temperature and pressure (STP, Standard Temperature and Pressure).

Standard temperature (T)

0°C or 273 K

Standard pressure (P)

1 atm (760 mm Hg)

STP

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Molar VolumeAt standard temperature and pressure (STP).• 1 mol of a gas occupies a volume of 22.4 L,

which is called its molar volume.

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

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The molar volume • At STP • Can be used to form conversion factors.

22.4 L and 1 mol

1 mol 22.4 L

Molar Volume as a Conversion Factor

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Guide to Using Molar Volume

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

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A. What is the volume at STP of 4.00 g CH4?

1) 5.59 L 2) 11.2 L 3) 44.8 L

B. How many grams of He are present in 8.00 L as

at STP?

1) 25.6 g 2) 0.357 g 3) 1.43 g

Learning Check

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A. 1) 5.59 L4.00 g CH4 x 1 mol CH4 x 22.4 L (STP) = 5.59 L 16.04 g CH4 1 mol CH4

B. 3) 1.43 g8.00 L x 1 mol He x 4.003 g He = 1.43 g He 22.4 L 1 mol He

Solution

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AT STP, the density of gas can be calculated using the mass of the gas and its volume.

Density = Molar mass Molar volume

Density of a Gas at STP

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Density of a Gas

Calculate the density in g/L of O2 gas at STP. At STP, we know that 1 mol O2 (32.00 g) occupies a volume of 22.4 L.

Density O2 at STP = 32.00 g O2 = 1.43 g/L

22.4 L (STP)