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Chapter 13:
Gases
What Are Gases?
 Gases
have mass
 Much less
compared to
liquids and solids
Properties of Gases
 Gases
are easy
to compress
 Liquids and
solids are
difficult to
compress
What Are Gases?
 Gases
fill their
containers
completely
 Liquids and
solids have fixed
volumes
What Are Gases?
 Gases
move very quickly
 Movement of gases is called diffusion
What Are Gases?
 Gases
exert pressure
 Pressure due to collisions
Kinetic Molecular Theory
The behavior of gas is
explained by the Kinetic –
Molecular theory of matter
1. Gases consist of particles
that have mass
 Gases are either
monoatomic, diatomic,
or polyatomic.

Kinetic Molecular Theory
2. There are large distances between
gas particles
– Volume of particles is assumed to
be zero compared to the total
volume
3. Gas particles are in constant random
motion
Kinetic Molecular Theory
4. Collisions of gas particles with other
particles or with the walls of the
container are completely elastic.
Elastic collision
Kinetic Molecular Theory
5. Kinetic energy of gas depends upon
temperature
– High temperature, high KE
– Low temperature, low KE
6. Gas particles exert no attractive
forces between one another
www.falstad.com/gas/
Describing Behavior of Gases
Depends upon 4 variables
1. Amount of Gas (n) – Measured in
Moles (mol)
2. Volume (V) – Measure in Liters (L)
3. Temperature (T) – Measured in Kelvin
(K)
4. Pressure (P) – Measured in Pascals
(Pa)

Pressure

Pressure is the amount of force per
given area
P = F/A
 One newton of force per square
meter is the Pascal
 Often measure in kPa

Atmospheric Pressure

Pressure exerted by the
atmosphere
Atmospheric Pressure
Air Pressure measured by a barometer
 Measured in mmHg

Atmospheric Pressure

Air pressure changes with altitude
Atmospheric Pressure

Air pressure changes due to the
heating by the sun
Manometer
Enclosed gases may be measured by a
manometer
 Pressure of gas = height + air pressure

Units of Pressure
 1013.25
millibars (mb) =
101.325 kPa = 1 atmosphere
(atm) = 14.7 pounds per in2
(psi) = 760 mm of Hg (torr)
Practice
 High
Pressure 1085 mb in Mongolia
(2001) Convert to kPa
 Low Pressure 870 mb in a Pacific
Typhoon (1979) Convert to Atm
Boyle’s Law
 The
pressure and volume of a sample
of gas are inversely proportional at
constant temperature
 “spring of air”
 P1V1=P2V2
Practice
A
weather balloon contains 150. L of
gas, internal gas pressure is 1.0 atm.
Atmospheric pressure at 41 km is 0.4
atm. Calculate the new volume of the
balloon.
 2.00 L of a gas is at 740.0 mmHg
pressure. What is its volume at
standard pressure?
Charles’s Law
 At
constant pressure, the volume of a
fixed amount of gas is directly
proportional to its temperature.
 Helped determine absolute zero
 V1/T1=V2/T2
 All temperatures must be converted to
Kelvin
Practice
 What
will be the volume of a gas
sample at 355 K if its volume at 273 K
is 8.57 Liters?
 At constant pressure a gas can be
used as a thermometer. A sample of
gas has a volume of 1 L at -18 0C.
What is the temperature if the
volume of the same gas is changed to
0.45 Liters?
Avogadro's Law
 Avogadro’s
Law states equal
volumes of gases at the same
temperature and pressure contain an
equal number of particles.
 Molar volume 22.4 L at STP.
Dalton’s Law
 Dalton’s
Law of Partial Pressure
states that the sum of the partial
pressures of all of the components in
a gas mixture is equal to the total
pressure of the gas mixture.
 PT = Pa + Pb + Pc ….
 What
Practice
is atmospheric pressure if the
partial pressures of N2, O2, and Ar are
604.5 mm Hg, 162.8 mm Hg, and 0.5
mm Hg respectively?
 The gases of carbon dioxide, oxygen,
nitrogen, neon, and krypton are mixed
in a container. All gases have the same
partial pressure and the total pressure
is 33,500 Pa. What is the partial
pressure of nitrogen?
Ideal Gas vs. Real Gas
 An
Ideal Gas represents a gas as
described by the kinetic-molecular
theory.
 Assumes no interaction between gas
molecules
 All gases are Real Gas
 An ideal gas behaves >95% like a real
gas.
 Real Gases deviate from ideal gases at
low temperatures and high pressures.
Ideal Gas Law

The ideal gas law combines all variables
involved in a gas.
PV = nRT
R is a constant
 R = 0.0821 atm-L/mol-K
 Temp must be in K
 Pressure must be in Atm
 Volume must be in Liters

Practice
 How
many moles of a gas at 100 oC
does it take to fill a 1.00 Liter flask to a
pressure of 1.50 atm?
 What is the volume occupied by 9.45 g
of C2H2 at STP?
 How many kilograms of oxygen gas are
contained in a sample that occupies
505 L at 675 kPa and -15 oC?