1 Chapter 11 Gases 11.7 Volume and Moles (Avogadro’s Law) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

2 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. V 1 = V 2 n 1 n 2 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

3 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

4 Solution 3) 2.4 L Conditions 1Conditions 2 V 1 = 1.5 LV 2 = ??? n 1 = 0.75 mol Hen 2 = 1.2 mol He V 2 = V 1 n 2 n 1 V 2 = 1.5 L x 1.2 mol He = 2.4 L 0.75 mol He

5 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

6 Molar Volume At 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

7 The molar volume At STP Can be used to form conversion factors L and 1 mol 1 mol 22.4 L Molar Volume as a Conversion Factor

8 Guide to Using Molar Volume Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

9 A. What is the volume at STP of 4.00 g CH 4 ? 1) 5.59 L2) 11.2 L3) 44.8 L B. How many grams of He are present in 8.00 L as at STP? 1) 25.6 g2) g3) 1.43 g Learning Check

10 A. 1) 5.59 L 4.00 g CH 4 x 1 mol CH 4 x 22.4 L (STP) = 5.59 L g CH 4 1 mol CH 4 B. 3) 1.43 g 8.00 L x 1 mol He x g He = 1.43 g He 22.4 L 1 mol He Solution

11 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

12 Density of a Gas Calculate the density in g/L of O 2 gas at STP. At STP, we know that 1 mol O 2 (32.00 g) occupies a volume of 22.4 L. Density O 2 at STP = g O 2 = 1.43 g/L 22.4 L (STP)