12.1:
Avogadro’s Law
and Molar Volume
Law of Combining Volumes
Gases always react to produce products in
whole-number ratios
 Example:
nitrogen gas and hydrogen gas
produce ammonia gas
N2 (g) + 3 H2 (g)  2 NH3 (g)
Avogadro’s Law
 Avogadro
discovered that the volume of a gas
is directly related to the amount (mols) of
gas.
 Temperature
and pressure remain constant.
V∝ n
𝑉1 𝑉2
=
𝑛1 𝑛2
V
𝑉
= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝑛
n
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) (56L)
GIVEN:
Step 1: Find moles of helium
Step 2: Find the total amount of helium
Step 3: Solve for the final volume
Molar Volume
The volume occupied by one mole of a gas.
It is the same for all gases
1
mole of any gas at STP occupies 22.4 L
1
mole of any gas at SATP occupies 24.8 L
Example 1: A party balloon has 2.50 mol of
helium gas in it at STP. What is the volume of
the balloon? (56L)
Example 2: A sample of helium at SATP has a
mass of 32.0 g. What volume does this mass
of gas occupy? (198L)
12.2
Ideal Gases and the
Ideal Gas Law
Properties of an Ideal Gas
 Particles
travel in straight lines
 Particles
have no attractive or repulsive forces
between them
 Collisions
with container walls or other particles are
perfectly elastic (there is no loss of kinetic energy)
 The
volumes of the ideal gas is essentially zero
compared to the volume of the container
 Do
not condense into liquids when cooled
The Ideal Gas Law
A
combination of Charles’ Law, Avogadro’s Law, and
Boyle’s Law
 Pressure
and volume of a gas are directly
proportional to the amount and the absolute
temperature of the gas
 Universal
Gas Constant (R) – a constant that relates
pressure, volume, amount, and temperature of an
ideal gas.
R
= 8.314 kPa·L/mol·K

Example 1: The Goodyear blimp has a volume of 2.5 x 107 L and usually
operates with the gas at a temperature of 12oC and a pressure of 112 kPa.
What amount of helium does the blimp contain? What is the mass of this
amount of helium? (4.7 x 106g)

Example 2: Calculate the volume of 32.4 g of nitrogen gas, N2(g) in a container
at 25oC and 96.4 kPa. (29.7L)

Example 3: 10.24 g of a pure gas occupies 2.10 L at 123oC and 99.7kPa.
Calculate the molar mass of this gas. (161g/mol)
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