1. Basic Chemistry Chapter 11 Gases Chapter 11 Lecture
Fourth Edition
Chapter Gases
11.7 Volume and Moles (Avogadro’s Law)
Learning Goal Use Avogadro’s law to determine the amount or volume of a gas when the pressure and temperature are constant.

2. Avogadro’s Law
Avogadro’s law
was formulated by Amedeo Avogadro in 1811
states that the volume of a gas is directly related to the number of moles of gas when temperature and pressure are held constant

3. Avogadro’s Law
The volume of a gas is directly related to the number of moles of the gas. If the number of moles is doubled, the volume must double at constant temperature and pressure.

4. Learning Check
If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure?

5. Step 1 Organize the data in a table of initial and final conditions.
Solution
If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure?
Step 1 Organize the data in a table of initial and final conditions.
Conditions 1
Conditions 2
V1 = 1.5 L
V2 = ?
n1 = 0.75 mole
n2 = 1.2 moles

6. Solution
If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure?
Step 2 Rearrange the gas law equation to solve for the unknown quantity.

7. Solution
If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure?
Step 3 Substitute values into gas law equation and calculate.

8. STP and Molar Volume
The volumes of gases can be compared at STP (standard temperature and pressure) when they have the same number of moles of gas.
Standard temperature is exactly 0 °C (273 K).
Standard pressure is exactly 1 atm (760 mmHg).

9. STP and Molar Volume
At STP, 1 mole of any gas occupies a volume of L, approximately equal to the volume of about three basketballs.

10. STP and Molar Volume
When a gas is at STP conditions (0 °C and 1 atm), its molar volume can be written as a conversion factor and used to convert between moles of gas and its volume, in liters.

11. Guide to Using Molar Volume

12. What is the volume occupied by 2.75 moles of N2 gas at STP?
Solution
What is the volume occupied by 2.75 moles of N2 gas at STP?
Step 1 State given and needed quantities.
Given: moles N2 at STP
Need: liters of N2 at STP
Step 2 Write a plan to calculate the needed quantity.
Molar Volume
moles of N2
liters of N2

13. Solution
What is the volume occupied by 2.75 moles of N2 gas at STP?
Step 3 Write the equalities and conversion factors.

14. Solution
What is the volume occupied by 2.75 moles of N2 gas at STP?
Step 4 Set up the problem with factors to cancel units.

15. Density at STP
At STP, the density of a gas is calculated using the molar mass of the gas and its molar volume.
Balloons rise in the air because helium is less dense than air.

16. Learning Check
Calculate the density in g/L of O2 gas at STP.

17. Solution
Calculate the density in g/L of O2 gas at STP.
Step 1 State given and needed quantities.
Given: O2 gas at STP
Need: density of O2 at STP
Step 2 Write a plan to calculate the needed quantity.

18. Solution
Calculate the density in g/L of O2 gas at STP.
Step 3 Write the equalities and conversion factors.

19. Solution
Calculate the density in g/L of O2 gas at STP.
Step 4 Set up the problem with factors to cancel units.