C H E M I
S T R Y
Chapter 9
Gases: Their Properties and Behavior
STOICHIOMETRIC RELATIONSHIPS WITH GASES
Methanol (CH3OH) can be synthesized by the following
reaction:
CO(g) + 2 H2(g) -- > CH3OH(g)
What volume (in Liters) of hydrogen gas, measured at a
temperature of 355K and a pressure of 738 mmHg, is
required to synthesize 35.7 g of methanol?
EXAMPLES

Determine the mass of NaN3 required for an air
bag to produce 100.0L of N2 gas at 85.0oC and
1.00 atm according to the equation
2 NaN3(s) -- > 2Na(s) + 3N2(g)
PARTIAL PRESSURE AND DALTON’S LAW
Dalton’s Law of Partial Pressures: The total pressure exerted by a
mixture of gases in a container at constant V and T is equal to the
sum of the pressures of each individual gas in the container.
Ptotal = P1 + P2 + … + PN
Moles of component
Mole Fraction (X) =
Total moles in mixture
Xi =
ni
ntotal
or
Xi =
Pi
Ptotal
EXAMPLES

A 1.00 L vessel contains 0.215 moles of N2 gas
and 0.0118 moles of H2 gas at 25.5oC.
Determine moles fraction of each gas
 Determine the partial pressure of each gas
 Determine the total pressure

EXAMPLES

Determine the number of moles of each gas
present in a mixture of CH4 and C2H6 in a 2.00L
vessel at 25.0oC and 1.50 atm, given that the
partial pressure of CH4 is 0.39 atm
THE KINETIC-MOLECULAR THEORY OF GASES
1.
A gas consists of tiny particles, either atoms or molecules,
moving about at random.
2.
The volume of the particles themselves is negligible compared
with the total volume of the gas; most of the volume of a gas is
empty space.
3.
The gas particles act independently of one another; there are no
attractive or repulsive forces between particles.
4.
Collisions of the gas particles, either with other particles or
with the walls of a container, are elastic (constant temperature).
5.
The average kinetic energy of the gas particles is
proportional to the Kelvin temperature of the sample.
THE KINETIC-MOLECULAR THEORY OF GASES
molar
mass
average
speed
THE KINETIC-MOLECULAR THEORY OF GASES
GRAHAM’S LAW: DIFFUSION AND EFFUSION OF
GASES
Diffusion: The mixing of different gases by molecular motion with
frequent molecular collisions.
GRAHAM’S LAW: DIFFUSION AND EFFUSION OF
GASES
Effusion: The escape of a gas through a pinhole into a vacuum
without molecular collisions.
Graham’s Law:
Rate a
1
m
GRAHAM’S LAW: DIFFUSION AND
EFFUSION OF GASES

In comparing two gases at the same temperature and
pressure
Rate 1
Rate 2
=
√m2
√m1
EXAMPLE


Determine how much faster Helium atoms
moves, on average, than a carbon dioxide
molecule at the same temperature
Determine the molar mass and identity of a gas
that moves 4.67 times as fast as CO2
THE BEHAVIOR OF REAL GASES
The volume of a real gas is larger than predicted
by the ideal gas law.
THE BEHAVIOR OF REAL GASES
Attractive forces between particles become more important
at higher pressures.