Download 12.1 Avogadro`s Law and Molar Volume

CHAPTER 12: GAS LAWS, GAS MIXTURES, AND GAS REACTIONS
12.1 AVOGADRO’S LAW AND MOLAR VOLUME
GAY-LUSSAC’S LAW OF COMBINING VOLUMES
When gases react, volumes of gaseous reactants and products of chemical reaction (at equal
temperatures and pressures) are in simple whole number ratios.
Example: When hydrogen and oxygen come together to produce water, 2 unit volumes of
hydrogen and 1 unit volume of oxygen results in 2 unit volumes of water.
Mole Ratio
Volume Ratio
2 H2
2 mol
2 vol units
+
O2
 2 H2O
:
1 mol
: 2 mol
: 1 vol units : 2 vol units
AVOGADRO’S LAW

Equal volumes of gases at the same temperature and pressure contain an equal number of
molecules.

Consider the following example: A container with 1.0 L of H2 at STP has the same number of
molecules as a container with 1.0 L of N2 at STP, even though the mass of nitrogen is 14
times greater. The pressure is directly related to the number of particles in each container
of gas. So if each container had the same amount of pressure, there would be no difference
in the number of particles.
Mathematically,
nV
or
n = kV
or
n1 = n2
V1 V2
Sample Problem 1:
A balloon contains 2.0 L of helium gas at STP. How many moles of helium are present?
G: n1 = 1.00 mol
V1 = 22.4 L
V2 = 2.0 L
R: n2 = ?
A: n1 = n2
V1 V2
n2 = n1 x V2
V1
S: n2 = (1.00 mol)(2.0 L) / (22.4 L)
= 0.089 mol
S: Therefore, 0.089 mol He are present.
Sample Problem 2:
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?
MOLAR VOLUME
Molar Volume is the volume occupied by one mole of a gas. It is the same value for all gases.
The molar volume for a gas a STP is 22.4 L. The molar volume of a gas at SATP is 24.8 L.
Therefore at STP 1 mol = 22.4 L and at SATP 1 mol = 24.8 L
Sample Problem 3:
A sample of helium at SATP has a mass of 32.0 g. What volume does this mass of gas occupy?
Read Page 576-581 and Do questions 1-2, 3ace, 4, 6