Dr. Saidane
Chem. 152
Gases
Skills you should have mastered
Conceptual
1. Describe the kinetic theory and show how it accounts for the major characteristics
of gases.
2. Identify different units of pressure.
3. State Boyle’s law and explain how volume and pressure are related at constant
temperature.
4. State Charles’s law and explain how volume and temperature are related at constant
pressure.
5. State Gay-Lussac’s law and explain how temperature and pressure are related at
constant volume.
6. State Dalton’s law of partial pressures.
7. State the ideal gas law.
Problem-solving
1. Combine the gas laws to solve problems involving multiple changes.
2. Use Dalton’s law to solve problems involving mixtures of gases.
3. Use the ideal gas law to solve problems involving multiple changes.
4. Solve molecular mass, density, and Stoichiometry problems using the gas laws.

A gas is a fluid state of matter that fills the container it occupies and can be easily
compressed.

There are 11 elements that are gas under normal conditions. They all lie toward
the upper right of the periodic table. Because gases expand quickly to fill their
container, they do not interact strongly.

Gases are highly compressible which suggests that there is a lot of empty space
between the molecules.

Gases fill rapidly their container, which suggests that the molecules are moving
very rapidly.
MOLECULAR MOTION
The kinetic Model of an Ideal Gas
The kinetic model of an ideal gas is based on four assumptions:
1. A gas consists of a collection of molecules in continuous random motion.
2. Gas molecules are infinitely small.
3. Gas particles move in straight lines until they collide.
4. The molecules do not influence one another except during collisions.
The collisions change the speed and direction of the molecules, which gives an
average speed of gas molecules.
The temperature of a gas is a measure of the
average speed of its molecules:
The higher the temperature, the faster the
molecules travel on average.
MOLECULAR CHARACTER OF GASES
The Ideal Gas
The properties of a gas indicate that molecular volumes and molecular forces
(interactions) are small. When they are negligibly small, each molecule is completely
independent of every other molecule, and the gas is said to be ideal.
An ideal gas is defined as: A gas which both the volume of molecules and the forces
between the molecules are so small that they have no effect on the behavior of the
gas.
Pressure
The pressure of a gas arises from the collisions that its molecules make with the walls
of the container. The storm of collisions exerts an almost steady force on the walls.
The pressure of a gas is the force of collision a gas exerts divided by the area
subjected to the force. P = F/S.
The units for reporting pressure are Torr and atmospheres (1 atm = 760 Torr). The SI
unit is Pascal (1 Pa = 1 kg/m.s2).
A barometer is used to measure the pressure of the atmosphere. At sea level, the
pressure of the atmosphere is close to 1 atm.
THE GAS LAWS
The properties of gases were studied and resulted in several gas laws.
Boyle’s Law
Boyle observed that when he compressed a fixed amount of gas at constant
temperature, the pressure of the gas increased.
When the volume of a gas is
decreased, there are more molecules in a given volume, so there are collisions with
the walls. Because the impact on the walls increases, so does the pressure. The
implication of this result is that for a fixed amount of gas at constant temperature,
pressure is inversely proportional to volume.
P = constant / V
Charles’s Law
Charles and Gay-Lussac found that the volume of a gas in a container fitted
with a movable wall (to keep the pressure constant) increases as its temperature is
raised. When the temperature of a gas is increased and it is free to change its volume
at constant pressure (as depicted by the constant weight acting on the piston), the
volume increases. The implication of this result is that, for a fixed amount of gas
under constant pressure, the volume varies linearly with the temperature.
V = constant x T
A similar expression summarizes the results of measuring the variation of
pressure of a sample of gas in a container of fixed volume. The pressure of a fixed
amount of gas in a vessel of constant volume is proportional to the absolute
temperature. It is found that the pressure varies linearly with temperature and that the
data extrapolate to zero pressure at –273C.
P = Constant x T
Avogadro’s Principle
The volume occupied by a sample of gas at constant pressure and temperature
is proportional to the number of moles of molecules present.
V = Constant x n.
The Ideal Gas Law
The observations made by Boyle, Charles, Gay-Lussac, and Avogadro have been
combined into one simple relation between the pressure (P), volume (V), temperature
(T), and the number of moles (n) of a gas. This relation is called the ideal gas law:
PV = n RT, where R is a constant called the gas constant.
A gas that obeys this law under all conditions is called an ideal gas.
Molar Volume
The molar volume of an ideal gas at 1 atm and 0C is 22.4 L/mol and at 25C
is about 24 L/mol. The molar volume (at the specified temperature and pressure) is
used to convert the amount of reactant or product in a chemical reaction into a volume
of gas.
Gas Density
The densities of gases increase with increasing pressure and decreasing
temperature, and are proportional to the molar mass.
PV = n RT => P/RT = n/V = m/ MMxV = d/ MM => d = Px MM / RT
Mixtures of Gases
John Dalton showed how to calculate the pressure of a mixture of gases. He
called the partial pressure of each gas, the pressure the gas would exert if it occupied
the container alone. Dalton the described the behavior of gaseous mixtures by his law
of partial pressures: The total pressure of a mixture of gases is the sum of the partial
pressures of its components. A mixture containing the gases A, B, and C, the total
Pressure
PT =PA + PB + PC.