Gases
Chapter 13
Page 298
Gases

Properties:
Gases are fluids because their molecules/atoms
can flow
 Gases have low density
 Highly compressible – their volume can be
reduced
 Gases will completely fill their container

Gas Pressure
PRESSURE is a force exerted by the
substance per unit area on another
substance.
GAS PRESSURE is the force that the
gas exerts on the walls of its
container.
A balloon expands because the
pressure of the gas molecules is
greater than the pressure of the gas
molecules on the outside.
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html
Gas Pressure
Gas (air) on the outside of the
balloon is exerting pressure
onto surface of the balloon.
The atmospheric pressure
outside a balloon, PA, is the
impact of moving gas
molecules as they collide with
the skin of the balloon.
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html
Atmospheric Pressure
If you could measure
the weight of a
column of air above
the surface of the
Earth, it would
be14.70 lbs per
square inch!
This is also known
as 1 atmosphere or
1 atm
Atmospheric Pressure
As you descend
toward earth, the
atmosphere is denser
and the pressure is
higher
 When flying your ears
may “Pop” due to the
change in pressure

Measuring Pressure
Atmospheric Pressure is measured
using a barometer
This picture shows a mercury
barometer. As the atmospheric
pressure presses down on the mercury
in the pan, the mercury is forced into
the tube. The height of the mercury is
measured to obtain the pressure.
Measuring Pressure

Manometers
A manometer is a Ushaped tube full of a liquid
(usually mercury). As the
pressure is changed on one
side of the tube, the
mercury is forced up or
down accordingly.
The change in height of the
mercury (h) indicates the
change in pressure
Manometers
Animation
 http://www.chem.iastate.edu/group/Greenbo
we/sections/projectfolder/flashfiles/gaslaw/m
anometer4-1.html

Pressure Conversions
There are many units to express pressure:
- pounds per square inch (tire pressure)
- atmospheres
- torr
- pascals
- mm of Hg
- bars
We need to be able to convert between units.
Converting between units
 Use dimensional analysis! 
Ex: Convert the pressure of 1.000 atm to mm of
mercury
Conversion factor: 101325 Pa and 1 mm Hg
1 atm
133.322 Pa
Calculation:
1.000 atm x 101325 Pa x 1 mm Hg = 760 mm Hg
1 atm
133.322 Pa
Gases – Converting Between
Units
Unit
Abbreviation
Conversion Factor
Atmosphere
atm
1 atm = 101.325 kPa
Millimeters of Hg
mmHg
760 mm Hg = 1 atm
Torr
torr
760 torr = 1 atm
Pascal
Pa
101.325 kPa = 101325
Pa
Millibar
bar
1013.2 millibar = 1
atm
Pressure Conversions

Practice

The pressure of carbon dioxide is 72.7 atm.
What is this value in units of kilopascals?

The pressure of water vapor at 50 deg C is
12.33kPa. What is this value in mm of Hg?
Variables in Gas Laws

Whenever we discuss gas laws, we are
interested in 4 variables:
Number of moles of gas
 Volume of gas
 Pressure of gas
 Temperature of gas

First Gas Law: Boyle’s Law

Pressure-Volume Relationships
Boyle’s Law
Based on the following facts:
1. Gases can be compressed
2. Gases exert pressure
Boyle found that:
As volume decreases, the concentration, and
therefore the pressure, increases
Boyle’s
As volume decreases, the concentration, and
therefore the pressure, increases
Gases

Boyle’s Law States:
For a fixed amount of gas at a constant
temperature: as the volume of the gas decreases
the pressure increases
 We can use the following equation to calculate
changes in pressure or volume

Boyle’s Law
A graph of Boyle’s
Law shows the
relationship
between pressure
and volume is
inversely
proportional: as
one variable
increases, the
other decreases
Boyle’s Law
http://www.grc.nasa.gov/WWW/K12/airplane/aboyle.html
Animation of Boyle’s Law
Boyle’s Law

Example:

A given sample of gas occupies 523 mL at 760
torr. The pressure is increased to 1.97 atm,
while the temperature remains the same. What
is the new volume of gas?
Second Gas Law: Charles’ Law

Temperature-Volume
Relationships

Charles’ Law
 For a fixed amount of gas at a
constant pressure, the
volume of a gas increases as
the temperature of the gas
increases
For a fixed amount of gas at a constant pressure, the
volume of a gas increases as the temperature of the
gas increases
Charles’ Law Graph
The graph of Charles’
Law is a straight line
(linear).
It shows the relationship
between temperature and
volume is directly
proportional: as one
variable increases or
decreases, so does the
other
Charles’ Law Formula:
Remember! ALWAYS USE
KELVIN when dealing with
temperature in gas laws! All
°C temperatures MUST be
converted to Kelvin!!!
Charles’ Law

http://wright.nasa.gov/airplane/aglussac.html
Animation of Charles’ Law
Charles’ Law

Example:

A balloon is inflated to 665 mL volume at 27 deg C.
It is immersed in a dry-ice bath at -78.5 deg C.
What is its new volume, assuming the pressure
remains constant?
Third Gas Law: Gay-Lussac’s
Law

Temperature-Pressure
Relationships
Gay-Lussac’s Law:
 The pressure of a gas at
a constant volume is
directly proportional to
the absolute temperature
(temperature in Kelvin)

Gay-Lussac’s Animation

http://www.chm.davidson.edu/ChemistryAppl
ets/KineticMolecularTheory/PT.html
Gay-Lussac’s Law Formula:
At constant volume:
P1 = P2
T1
T2
Avogadro’s Law

In 1811, Avogadro proposed
that equal volumes of all
gases, under the same
conditions, have the same
number of particles.
Avogadro’s Law
We know volume of a gas can change with
temperature and pressure, but what about
the number of molecules?
 Through Avogadro’s observations, the
following has been defined:
 1 mole of any gas at STP (0°C and 1 atm)
occupies 22.41 L


The mass of 22.41L at STP is the Molecular
Mass of the gas
Combined Gas Law

When you take Boyles, Charles’, GayLussac’s and Avogadro’s Laws and combine
them, you get the COMBINED GAS LAW
This law is used to solve problems where
pressure, volume and temperature of a gas
vary with a constant molar quantity of the gas
Combined Gas Law

Example:

A sample of hydrogen gas has a volume of 65.0
mL at a pressure of 0.992 atm and a
temperature of 16 deg C. What volume will be
hydrogen occupy at 0.984 atm and 25 deg C?
Dalton’s Law of Partial Pressures

John Dalton showed that in a mixture of
gases, each gas exerts a certain pressure as
if it were alone with no other gases with it.
This is called “partial pressure”
 Equation:

Pt
= P1 + P2 + P3…………
Dalton’s Law of Partial Pressures
Dalton’s Law of Partial Pressures

Example:

A 2.5 L flask at 15 deg C contains a mixture of
three gases: N2, He, and Ne. The partial
pressures are: N2 = 0.32 atm, He = 0.15 atm
and 0.42 atm for Ne. What is the total
pressure of the system?
Gases – Dalton’s Law of Partial
Pressures – Mole Fraction

As Dalton’s Law tells us, if a number of
gases are mixed, each contributes to the
pressure in its vessel.

To figure out how much pressure one gas
is contributing, we need to find:
1. Mole Fraction
 2. Its partial pressure.

Gases – Mole Fraction/Partial
Pressure
To calculate mole fraction (X):
X? =
moles of gas of interest
total moles of gas in the mixture
To calculate the partial pressure of that gas:
Px = X?Ptotal
Gases – Mole Fraction/Partial
Pressure

A mixture of gases contains 4.46 moles of
neon (Ne), 0.74 moles of argon (Ar) and
2.15 moles of xenon (Xe). Calculate the
partial pressure of Ne.
Gas Simulator

http://intro.chem.okstate.edu/1314F00/Labor
atory/GLP.htm