Molecular Composition
of Gases
Volume-Mass
Relationships of Gases
Objectives:
1. State Avogadro’s Law and explain its
significance.
2. Define the standard molar volume of a gas
and use it to calculate gas mass and volume.
3. Use molar volume to calculate the density
and molar mass of a gas at STP.
Volumes of Reacting Gases
Gay-Lussac’s Law of Combining Volumes of Gases
1808 – At constant temperature and pressure, the
volumes of gaseous reactants and products can be
expressed as ratios of small whole numbers
Hydrogen + Oxygen  water vapor
2L
1L
2L
Nitrogen + Hydrogen  ammonia
1L
3L
2L
Avogadro’s Law
• Equal volumes of gases
at the same temperature
and pressure contain
equal numbers of
molecules.
• Explained Gay-Lussac’s
observations.
Remember:
V
k
n
Avogadro’s Law
N2 + 3 H2  2 NH3
1L
3L
2L
1 molecule 3 molecules 2 molecules
1 mol 3 mol 2 mol
Avogadro’s Law
1. Ozone can be formed by the following reaction:
3 O2  2 O 3
If both gases are at constant temp. and pressure…
a) How many molecules of ozone are formed from the
reaction of 24 oxygen molecules?
b) How many moles of oxygen are required to produce
24 moles of ozone?
c) How many Liters of ozone are produced from 12 L
of oxygen
2. How many liters of chlorine at STP will react with 35
L O2 at STP? Use the following reaction.
2 Cl2 + 7 O2  2 Cl2O7
Molar Volume of Gases
At STP…….
1 mole of a gas = 22.4 L
• This conversion factor can be used with molar
mass to relate mass, moles, and volume of a gas.
Molar Volume of Gases
1. At STP, what is the volume of 7.08 mol of nitrogen
gas?
2. A sample of hydrogen gas occupies 14.1 L at STP.
How many moles of the gas are present?
3. What is the mass of 1.33 x 104 mL of oxygen gas at
STP?
4. What is the volume of 77.0 g of nitrogen dioxide gas
at STP
5. At STP, 3 L of chlorine is produced during a
chemical reaction. What is the mass of this gas?