12.1
Avogadro’s Law and
Molar Volume
The Law of Combining Volumes
• Water can be decomposed into hydrogen and
oxygen using electricity
2H2O(l)  2H2(g) + O2(g)
• H2O consists of 2 H atoms and 1 O atom.
• When water decomposes twice the volume of
hydrogen as oxygen is produced
The Law of Combining Volumes
• Proposed by Joseph Gay-Lussac
• The Law of Combining Volumes: When
measured at the same temperature and
pressure, volumes of gaseous reactants and
products of chemical reactions are always in
simple ratios of whole numbers
• Example: Combining 1.0 unit of H2(g) and
Cl2(g) produces 2.0 units of HCl(g)
The Law of Combining Volumes
• Ammonia gas can be produced by combining
nitrogen, N2(g), and hydrogen, H2(g)
N2(g) + 3H2(g)
 2NH3(g)
Coefficients:
Mole Ratio:
Volume Ratio:
1
1
1
3
3
3
2
2
2
• While Gay-Lussac Proposed the law, he was
unable to explain it
Avogadro’s Law
• Avogadro’s law: the volume (V) of a gas is
directly related to the amount (n) of the gas
when temperature and pressure remain
constant
V1/n1 = V2/n2
Where:
V is the Volume in L
n is the amount in mol
Avogadro’s Law
• Explains the law of combing volumes:
– Equal volumes of gases, at the same temperature
and pressure, contain the same number of
molecules.
– The mole ratios provided by the balanced
equation are also the ratios of volumes
– Example: 100mL of H2(g) at 25°C contains the
same number of molecules as 100mL of O2(g) at
25°C
Example
A balloon with a volume of 34.5 L is filled with
3.2 mol of helium gas. To what volume will the
balloon expand if another 8.0 g of helium is
added? (Assume that pressure and temperature
do not change.)
Molar Volume
• Molar Volume: the volume occupied by one
mole of a gas. It is the same for all gases.
– One mole of any gas at STP (0°C, 101.3kPa)
occupies 22.4L
– One mole of any gas at SATP (25°C, 100kPa)
occupies 24.8L
Example
1. A party balloon has 2.50 mol of helium gas in
it at STP. What is the volume of the
balloon?(at STP 1mol = 22.4L)
1. A sample of helium at SATP has a mass of
32.0 g. What volume does this mass of gas
occupy? (at SATP 1mol = 24.8L)
Homework
Read
Section 12.1
Questions
p. 579 #1-3
p. 580 #1-3
p. 581 #4