The Property of Gases –
Kinetic Molecular Theory
And Pressure
Kinetic Molecular Theory of Gases
• The word kinetic refers to motion.
• Kinetic energy is the energy an object has because of its
motion.
• Kinetic Molecular Theory makes assumptions about:
▫ Size
▫ Motion
▫ Energy of gas particles
1. According to the KMT all matter consists of tiny particles
that are in constant, random motion
• Move in a straight line until they collide with other particles or with
the walls of the container.
2. Gas particles are much smaller than the distances
between them. Most of a gas consists of empty space.

Because they are so far apart, there are no attractive or
repulsive forces between the gas molecules

The motion of one particle is independent of the motion of
other particles
3. No kinetic energy is lost when gas particles collide with
each other or with the walls of the container (elastic
collision)

The total amount of kinetic energy remains constant.
4. All gases have the same average kinetic energy at a
given temperature
 Temperature is a measure of average kinetic energy of particles
in a sample of matter.
 Kinetic energy and temperature are directly related
 The higher the temperature, the greater the kinetic energy
Absolute Zero
• The greater the atomic and molecular motion, the greater the
temperature is of a substance.
• If all atomic and molecular motion would stop, the temperature
would be at absolute zero (0 Kelvin or -273 oC)
• 273 + _____oC = _______Kelvin
Diffusion and Effusion
• Diffusion – describes the movement of one material
through another
▫ Particles diffuse from an area of high concentration to low
concentration
• Effusion – gas escapes through a tiny opening.
• The heavier the molecule, the slower it will effuse or diffuse
Diffusion and Effusion
Diffusion
Effusion
Pressure
• Pressure is the force per unit area
• Gas pressure is the force exerted by a gas per unit
surface area of an object.
 Gas pressure is the result of billions of collisions of billions of
gas molecules with an object
• Atmospheric pressure (air pressure) results from the
collisions of air molecules with objects.
 The air pressure at higher altitudes is slightly lower than at sea
level because the density of the Earth’s atmosphere decreases
as elevation increases.
• Vacuum - Empty space with no particles and no pressure
Measuring Pressure
• Barometer – an
instrument used to
measure atmospheric
pressure
Measuring Pressure
• Manometer – an
instrument used to
measure gas pressure in
a closed container
Units of Pressure and STP
• Average atmospheric pressure is 1 atm
• STP (Standard Temperature and Pressure)
▫ 1 atm and 0oC or 1 atm and 273 K
Conversion Factors for Pressure
1 atm = 760 torr = 760 mmHg = 101.3 kPa
Example 1
Convert 2.5 atm into torr, mmHg, kPa
2.5 atm
760 torr
1 atm
2.5 atm
760 mmHg
1 atm
2.5 atm
101.3 kPa
1 atm
= 1900 torr
= 1900 mmHg
= 250 kPa
Example 2
Convert 215 kPa into torr, mmHg, atm
215 kPa
760 torr
101.3 kPa
215 kPa
760 mmHg
101.3 kPa
215 kPa
1 atm
101.3 kPa
= 1610 torr
= 1610 mmHg
= 2.12 atm
Dalton’s Law of Partial Pressures
• Dalton’s law of partial pressures states that the total
pressure of a mixture of gases is equal to the sum of the
pressures of all the gases in the mixture.
Ptotal = P1 + P2 + P3 … + Pn
Example 1
A mixture of O2, CO2, and N2 has a total pressure of 0.97 atm.
What is the partial pressure of O2, if the partial pressure of CO2 is
0.70 atm, and the partial pressure of N2 is 0.12 atm?
Ptotal = PO2 + PCO2 + PN2
0.97 atm = PO2 + 0.70 atm + 0.12 atm
PO2 = 0.15 atm
Example 2
There is a mixture of CO2, O2, and CO in a container. What is the
total pressure if the pressure in atm if O2 is 0.563 atm, CO2 is 235
kPa, and CO is 455 torr?
Ptotal = PO2 + PCO2 + PCO
235 kPa
1 atm
101.3 kPa
= 2.32 atm
455 torr
1 atm
760 torr
= 0.599 atm
Ptotal = 0.563 atm + 2.32 atm + 0.599 atm
Ptotal = 3.48 atm
**You first
have to put
everything in
the same units!
COCl2
C2H2AsCl3
Cl3CNO2
C4H8Cl2S