Properties of
Gases: The
Air We Breathe
Chapter 6
What do we know about the air
we breathe?
Component
• Gas
composition of
dry air
• Water vapor
varies
– 0.1% in dry
desert
– 6% wet, humid
areas
Symbol
Volume
Nitrogen
N2
78.084%
Oxygen
O2
20.947%
Argon
Ar
0.934%
Carbon Dioxide
CO2
0.033%
Neon
Ne
18.2 parts per million
Helium
He
5.2 parts per million
Krypton
Kr
1.1 parts per million
Sulfur dioxide
SO2
1.0 parts per million
Methane
CH4
2.0 parts per million
Hydrogen
H2
0.5 parts per million
N2O
0.5 parts per million
Nitrous Oxide
99.998%
Xenon
Xe
0.09 parts per million
Ozone
O3
0.07 parts per million
NO2
0.02 parts per million
I2
0.01 parts per million
Nitrogen dioxide
Iodine
Carbon monoxide
CO
trace
Ammonia
NH3
trace
2
• Atmospheric
molecules under
the influences of
gravitational
forces.
• Air has mass.
(5.3 x 1018 kg)
• Weight = ma
(a = 9.81 m/s2)
Constituent
Symbol
Molecular
Weight
Fraction
by mass
Nitrogen
Oxygen
Argon
Carbon dioxide
Neon
Helium
Methane
Krypton
Nitrous oxide
Hydrogen
Ozone
N2
O2
Ar
CO2
Ne
He
CH4
Kr
N2O
H2
O3
28
32
40
44
20
4
16
84
44
2
48
75.5%
23.2 %
1.3 %
486 ppm
12 ppm
0.7 ppm
1.6 ppm
3.2 ppm
0.8 ppm
0.03 ppm
0.02 ppm
3
• Pressure = force per unit area
• Surface area of Earth = 5.1 x 1014 m2
average pressure exerted by air on Earth 
(5.3 x 1018 kg)(9.81 m/s 2 )
5.1 x 1014 m2
 1.0 x 105 N/m2
• 1 N/m2 = 1 Pascal (Pa)
– SI unit of pressure
– Standard atmospheric pressure = 101.325 kPa
– More common units
• 1 atm = 760 mm Hg = 760 torr
4
• As altitude increases,
–
–
–
–
Fewer gas molecules
Decrease in pressure
Decrease in density
Decrease in
temperature
5
Ear “Popping”
• Differences in pressure at ear drum
• Ear “pops” when the pressure is
equalized.
6
Boyle’s Law
• British chemist Robert Boyle (16271691)
• T, n constant
• PV = nRT = constant
P
constant
V
P
1
V
• Pressure is inversely proportional to
volume (1662).
7
Charles’s Law
• French scientist Jacques Charles
(1746-1823)
• P, n constant
nRT
V
 constant  T
P
V T
• Volume increases as T increases.
(1787).
8
Avogadro’s Law
• Italian chemist Amedeo Avogadro
(1776-1856)
• P, T constant
V
nRT
 n  constant
P
V n
• Volume of a gas increases with its
mass (1811).
• Same volumes of different gases
contain the same number of moles.
9
Amontons’s Law
• French physicist Guillaume Amonton
(1663-1705)
• n, V constant
nRT
P
 constant  T
V
P T
• Pressure increases with temperature
(1699).
Applying the principles
• Problem 6.46, page 301
11
Applying the principles
• Problem 6.74, page 302

12
Ideal Gas Law
• Ideal gas
– No interactions between molecules
– Random, independent motion
– V occupied by molecules >>> V of molecules
themselves
• Measurable properties
–
–
–
–
Mass
Pressure
Volume
Temperature
• Ideal gas law: PV = nRT
– n = number of moles
– T = Kelvin units
13
Applying the principles
• Problem 6.82, page 303
14
Standard Molar
Volume
• Standard temperature and pressure
(STP)
– 0C (273 K), 1 atm
nRT (1.00 mol)(0.082 06 L  atm)(273 K)
V

 22.4 L
P
1.00 atm mol  K
15
Gas Density
• At STP
molar mass
d
22.4 L
• At any T, P
P  molar mass
d
RT
16
Applying the principles
• Problem 6.92, page 303
17
Dalton’s Law
• English chemist and physicist John Dalton
(1766-1844)
• Applies to gaseous mixtures
• Each gas exerts it own pressure
independent of the other gases (1801).
Ptotal = p1 + p2 + ….
pgas = XgasPtotal
X1 
p = partial pressure
X = mole fraction
n1
n1  n2  n3    
18
Applying the principles
• Problem 6.106, page 304
19
Kinetic Molecular Theory
• Gas molecules are in constant, random
motion.
• No forces of attraction or repulsion
between gas molecules.
• Collisions are elastic (no energy
transfer).
• Average kinetic energy of gas
molecules is directly proportional to T.
• Average velocities of molecules are
inversely proportional to their masses.
– Heavier gas molecules mover slower than
lighter ones.
u rms 
3RT
M
20
Graham’s Law
• Effusion: loss of gas through a small
opening in container
• Diffusion: intermingling of two or
more gases
rate x

rate y
My
Mx
Real Gases
• Deviations from ideal behavior at
high P and low T
– Volume of gas molecules no longer
negligible to space they occupy
– Intermolecular attractions present
• van der Waals equation
n2a
nRT  (P  2 )(V  nb )
V
22
Boyle’s Law and Breathing
23
Toxins
• Botulism
• Cobra venom
• Spider venom
• Poison frogs
• Curare
24
Lung pressure
25
Collapsed Lung
• Pneumothorax
– Results from puncture into
the pleural cavity of the
chest wall or the lung.
– The collapse results from air
entering the pleural cavity,
as it moves from high
pressure to low pressure.
26
Diffusion of respiratory
gases
• At the lungs:
pO2 (lungs)  pO2 (blood)
pCO2 (lungs)  pCO2 (blood)
O2 flows from lungs
to blood and CO2
flows from blood to
lungs.
27
• At cells:
p O2 (cells)  p O2 (blood)
p CO2 (cells)  p CO2 (blood)
28
O2 passes
from blood into
cells, and CO2
passes from
cells into blood.