Gas Mixtures--Partial Pressure
 Many gases are actually mixtures of two or
more gases:
 air: O2, N2 , H2O, etc
 How do mixtures of gases behave?
Gas Mixtures--Partial Pressure
P= 6 psi
O2 (g)
P= 8 psi
N2 (g)
P= 9 psi
CO2 (g)
Gas Mixtures--Partial Pressure
P
What happens when you put all
three samples of gas together
into one container (the same
size container as each was in
alone)?
Gas Mixtures--Partial Pressure
 The gases form a
homogeneous mixture.
 The pressure in the
container increases.
 How do you know what
the new pressure will
be?
Gas Mixtures--Partial Pressure
 Each gas in a mixture behaves independently
of the other gases present.
 Each gas exerts its own pressure on the
container.
 PO = pressure exerted by O2
2
 PN
= pressure exerted by N2
2
 PCO
= pressure exerted by CO2
2
Gas Mixtures--Partial Pressure
 Partial pressure:
the pressure exerted by a
particular gas present in a mixture
 Dalton's Law of Partial Pressure:
The total
pressure of a mixture of gases equals the
sum of the pressures that each would exert
if it were present alone.
Ptotal = P1 + P2 + P3 + ………
Gas Mixtures--Partial Pressure
 Ptotal = PO
2
+ PN
+ PCO
2
2
 So for this example:
Ptotal = 6 psi + 8 psi + 9 psi
= 23 psi
Gas Mixtures--Partial Pressure
 In other words, at constant T and V,
 Ptotal depends only on the total number of
moles of gas present
 Ptotal is independent of the type (or types) of
gases present.
Gas Mixtures--Partial Pressure
 When describing a mixture of gases, it is
useful to know the relative amount of each type
of gas.
 Mole fraction (X):
the ratio of the number of
moles of one component compared to the total
number of moles in a mixture.
Gas Mixtures--Partial Pressure
 If a gas mixture contains 5.0 mol O2 (g), 3.0
mol H2O (g), and 12.0 mol N2 (g),
 XO
2
=
nO
2
ntotal
=
5.0 mol
20.0 mol
=
0.25
 On the exam, you must be able to calculate the
mole fraction of each component of a gas
mixture.
Gas Mixtures--Partial Pressure
 The partial pressure of a gas in a mixture can
be found:
PA = XA Ptotal
where PA = partial pressure of gas A
XA = mole fraction of gas A
Ptotal = total pressure of mixture
Gas Mixtures--Partial Pressure
Example: A mixture of gases contains 0.43 mol
N2, 0.28 mol H2, and 0.52 mol NH3. If the
total pressure of the mixture is 2.35 atm, what
is the partial pressure of H2?
Gas Mixtures--Partial Pressure
 Gases formed during a reaction are often
collected by displacing water from a container.
 The gas collected is a mixture that contains
water vapor and the gas that was formed.
 Ptotal = Pgas + Pwater
vapor
 The vapor pressure (partial pressure) of
water at various temperatures can be
found in Appendix B in your text).
Gas Mixtures--Partial Pressure
Example: The oxygen gas formed during the
decomposition of potassium chlorate was collected
by displacing the water in a gas measuring tube.
What is the partial pressure of O2 in the gas
collected if the total pressure of the gas was
745 torr at 25oC?
Molecular Effusion & Diffusion
 What happens to a helium filled balloon as it
sits around for several days?
Molecular Effusion & Diffusion
 What happens to a helium filled balloon as it
sits around for several days?
Molecular Effusion & Diffusion
 What happens to a helium filled balloon as it
sits around for several days?
Molecular Effusion & Diffusion
 What happens to a helium filled balloon as it
sits around for several days?
Molecular Effusion & Diffusion
 What happens to a helium filled balloon as it
sits around for several days?
Molecular Effusion & Diffusion
 Many materials such as rubber or plastics have
very small openings or pores through which
gases can pass.
 The escape of a gas molecule through a tiny
hole is called effusion.
Molecular Effusion & Diffusion
 The rate of effusion (r) of a gas is inversely
proportional to the square root of its molar
mass, M.
 Graham’s Law:
r1 = 1
M1
1/2
Molecular Effusion & Diffusion
A molecule with smaller molar mass escapes
faster than a molecule with higher molar mass.
Molecular Effusion & Diffusion
 Another phenomenon, diffusion, also depends on
the molar mass of the gas.
 Diffusion: the spreading of one substance
throughout another
 perfume molecules spreading throughout the
air in a room
Real Gases
 Real gases do not completely follow the ideal
gas law.
 In kinetic molecular theory, the following
assumptions are made:
 gas molecules occupy no space
 gas molecules have no attraction for each
other
Real Gases
 Neither assumption is correct.
 Real gas molecules have a finite volume.
 Real gas molecules do attract each other.
Real Gases
 The greatest deviation from ideal gas behavior
occurs at:
 high pressure
 higher density of gas molecules
 Molecules are closer together so:
 finite volume of gas molecules more
important
 attraction between molecules more
important
Real Gases
 Low temperature
 Attractive forces between molecules
becomes more important.
 Average kinetic energy decreases.
 Gas molecules have less energy to
overcome attractive forces.