Chapter 9 Objectives
Intro to gases, pressure
Boyle’s Law, Charles’ Law, Avogadro’s Law,
Dalton’s Law, Ideal Gas Law, Graham’s Law
Kinetic Molecular Theory
Deviations from ideal behavior and the van der
Waals equation

Intro to Gases
Gases will expand to fill the volume of their
containers
Mixtures of gases will mix evenly and completely
when confined in a container
Gases are compressible and have lower densities
than solids or liquids

Pressure
Pressure is defined as F/A
1 Pascal = 1 N/m
2
Instruments for measuring pressure often use
mercury because of its high density
STP
–
standard temperature and pressure
1 atm and 0° C
Hg
Hg is a liquid
at room
temperature
Atmospheric pressure

Units of Pressure
1 atm =
760 mm Hg
760 torr
101325 Pa
1.01325 bar
14.696 psi

Boyle’s Law
The Pressure-Volume Relationship
At constant T, the volume of a gas is inversely
proportional to the pressure
As volume decreases, pressure increases.
As
volume increases, pressure decreases
Since P
1
V
1
= constant and P
2
V
2
= constant,
P
1
V
1
= P
2
V
2
P
constant
V

A sample of Neon to be used in a neon sign has a
volume of 1.51 L at a pressure of 635 torr. Calculate the
volume of the gas after it is pumped into the glass tubes
of the sign, where it shows a pressure of 785 torr.
2
nd
:
what information is given?
1
st
:
what are we looking for?
3
rd
:
Check for the same units of pressure:
Example 1

4
th
: Solve for V
2
using Boyle’s Law
V
2
=
(V
1
x
P
1
)/P
2
=
(1.51 L x 635 torr)/785 torr
=
1.22 L

Temperature
A measure of the average translational kinetic
energy of gaseous molecules
The faster molecules move on average, the
greater their kinetic energy
more kinetic
energy means higher temperatures
KE = ½ mv
2
The temperature of a gas can be related to the
volume it occupies

Charles’s Law
The Temperature-Volume Relationship
At constant P, the volume of a gas increases
linearly with temperature
V = constant x T
As temperature increases, volumes increases.
As temperature decreases, volume decreases.
Since V
1
= constant x T
1
and V
2
= constant x T
2
,
2
2
1
1
T
V
T
V
HAVE to use
Kelvin temperatures!

Implications of Charles’s Law Data
Plotting V vs. T gives a straight
line
Extrapolating the plots for different
gases to V = 0 all gave the same
x-intercept of -273.15°C
Lord Kelvin named this absolute
zero and based his temperature
scale on it

Temperature is sometimes measured with a gas thermometer
by observing the change in the volume of the gas as the
temperature changes at constant pressure. The hydrogen in a
particular hydrogen gas thermometer has a volume of 150.0
cm
3
when immersed in a mixture of ice and water (0.00 °C).

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- Fall '19