Gas Properties and Laws
• Explains why gases act as they do.
Assumptions/Postulates of the theory
1. Gases are composed of small particles.
2. These particles are in constant, straight-line, and
random motion.
3. Collisions between these particles are elastic
with each other. Nor do they exert
attraction/repulsion on each other.
4. The average kinetic energy of the gas
molecule is proportional to the temperature.
Kinetic Molecular Theory: Basic Concepts
1. Volume: Gas molecules can be compressed
therefore one volume is able to occupy a
variety of volumes.
Ex. How much CO2 gas is in a coke can?
That same amount will fill a trash can.
2. Temperature: Since gases are “free”
molecules, their energy is important. The
temperature can determine how the gas
will be or act.
3. Pressure: This affects gases a lot more than the
other 2 states of matter. This can also define how
the gas will act.
Force is the mass being applied.
Pressure is the mass being applied in a defined
area. Pressure = Force/area
Atmospheric pressure: The amount of pressure
being exerted by the mass of a column of air over
1m2 of space.
Weather: Low pressure means it is cloudy
High pressure would be sunny
Gas pressure is how often the molecules of the gas
collide with the walls of its container.
Barometers: this device is used to measure
atmospheric pressure.
Simple Barometer: this is a glass
column going into a pool of mercury.
As the atmospheric pressure pushes
down on the Hg pool, it causes some
to rise or fall in the column.
When using a barometer, the mercury level is
measured in the column. Thus the unit is
mmHg
The unit Torr is the same as mmHg
The SI unit is the Pascal (Pa). But Kpa is more
commonly used.
For gas laws, the unit Atmospheres (atm) is used
often.
At sea level 1atm=101,325pa or 101.325kpa
760mmHg or 760torr
14.69lbs/in2
Manometer: is a device that is used to determine
the pressure of a gas inside of a container.
This device has a U-tube with a specific amount of
Hg in it.
The container of gas is hooked up to it and the gas is
allowed to hit the Hg.
The difference is the levels of Hg on both sides tells
you the pressure of the gas.
Closed ended: one end has a vacuum with a specific
amount of pressure in it.
Open ended: the atmospheric pressure is used.
Manometers
Open ended
Closed ended
Manometer variables: 1. Pressure of container
2. Pressure of atmosphere
or vacuum
3. Height difference of Hg
in tube
In a manometer problem; you need to know at least 2 of
3 variables.
You must also be sure that the pressures are in the same
unit as the mmHg. You may need to do conversions.
1. A soccer ball is attached to an open-ended
manometer. The mercury level in the manometer is
10.0 mm higher on the side attached to the ball than
on the side open to the atmosphere. Atmospheric
pressure has already been determined to be 770.0
mm Hg. What is the gas pressure in the ball?
2. A manometer contains a sample of
nitrogen gas at a pressure of 88.3 kPa.
The level of mercury in the U-tube is 12.8
mm lower on the end open to the
atmosphere. What is the atmospheric
pressure in torr?
If you have an empty bottle is it really empty? (cap is off)
Any gas trying to be collected into that bottle will
simply mix with the other gases in air.
Water displacement can be used to collect gases with
absence of air, but there will be a small amount of water
vapor.
When you mix gases, each have a certain amount of
pressure. This is called a Partial Pressure.
Dalton’s law of partial pressure is for the total pressure of gases in
a system.
Ptotal = Pa + Pb + Pc
http://www.wwnorton.com/chemistry/tutorials/interface.s
wf?chapter=chapter_08&folder=daltons_law
Boyle’s Law
• Boyle’ Law describes
the volume-pressure
realationship.
This is an inverse relationship: As you add
pressure, volume decreases.
Boyle’s law is expressed as: P1V1 = P2V2
Pressure units do not matter
For this to happen though, temperature must
be constant.
Boyle's Law .. Animated
If a bag of chips is filled with enough air to create a
1.5L bag at 101.3kpa in Kansas City. What volume
will the bag be if it is taken to Denver at a pressure of
89.98kpa?
Write Boyle’s law: P1V1 = P2V2
Next determine which numbers go together: 1.5L
and 101.3kpa then ?L and 89.98kpa
Finally substitute the numbers and solve for x:
(101.3kpa)(1.5L) = (89.98kpa)(?L)
X = 1.69L
A 2.3L balloon is filled on the ground at
103.456kpa. The balloon is then taken in a
plane that has a pressure of 67.8kpa, what
is the volume of the balloon?
A. 3.5L
B. 1.5L
C. 3049.7L
D. Can not be determined with this info
Kelvin scale of temperature: In the SI system,
Kelvin is used since it does not have negative
values.
Zero Kelvin is called absolute zero.
Zero Celsius = 273K
Charles’s law is the direct relationship
between temp and volume of a gas
As temp increases  Volume increases
Charles’s Law equation:
V1T2 = V2T1 ( T must be in Kelvin)
Charles and Gay-Lussac's Law Animated
38c would be ______K
A. -235
B. 311
C. 7
A volume of argon gas at the temperature
of -10c occupies a volume of 1L. What
Celsius temperature will the gas need to be
to occupy a volume of 2.2L?
1st you have to make sure all of your temperatures are in
Kelvin. So -10c + 273 = 263K
Next write out Charles’s equation: V1T2 = V2T1
Identify which temp and volume go together and make
them the 1’s. The other variable then becomes the 2’s.
So: (1L)(x) = (2.2L)(263K)
X = 578.6K
578.6K – 273 = 305.6c
A can of soda has a volume of 350ml at 25c,
what volume would it be if it was at 45c?
A. 630mL
B. 328.0mL
C. 194.4mL
D. 373.5mL
If you want a balloon to get bigger how do you
make it bigger? (pressure and temp are constant)
Avogadro’s law is the direct relationship of volume
to quantity of particles.
V = n where n = moles
So if you have 2 samples: V1n2 = V2n1
Increase particles animated
Gay-Lussac’s Law: Pressure-Temp
From the information you have learned; what is
the relationship going to be between pressure and
temperature? Direct or Indirect?
How will the equation look?
P1T2 = P2T1
(T must be in Kelvin)
Pressure-Temp animated
If a sample of CO2 had a pressure of
400torr at -7c, what pressure would it
have when heated to 60c?
A. 329torr
B. 500torr
C. -3429torr
D. no change
• This Law takes into account 3 variables
by combining Charles’s and Boyle’s laws.
P1V1 = P2V2
T1
T2
A gas occupies a volume of 20 L at a pressure of 5 atm and
a temperature of 500K. What will the volume be if both the
pressure is raised to 10 atm and temperature is changed to
250K?
rewrite to put in the variables:
A gas occupies a volume of 20 L (V1) at a pressure of 5 atm
(P1) and a temperature of 500K (T1). What will the volume
(V2) be if both the pressure is raised to 10 atm (P2) and
temperature is changed to 250K (T2)?
Substituting into the equation
P1 V1 / T1 = P2 V2 / T2
we get:
(5 atm ) (20 liter) / 500K = (10 atm) ( V2) / 250K
Solving for V2,
(5 atm) (20 L) (250 K) / ( 10 atm) (500 K) = V2 = 5 L
In order for a gas to “lift off” from the ground, it
must have a lower density that air.
2 ways to achieve low density
1. Choose a gas with a
low molar mass
2. Heat the gas
Diffusion is the mixing
of gases in a system.
Generally this is from
higher concentration to
lower.
Effusion is the escape of
a gas from a system
molecule by molecule
through a small hole,
usually between the
molecules of the
container.
Ideal Gas: this is a gas that follows the
postulates of the kinetic theory. Most gases
do this unless under extreme pressure or
extreme low temperature.
The ideal gas law is the combination of
Boyle, Charles, and Avogadro’s law.
It will take into account 4 variables:
Pressure, Temperature, Volume, and
Moles.
The ideal gas law is expressed as:
PV=nRT
P = pressure
n= moles
V = volume
T = temperature
R = ideal gas law constant
• R is the ideal gas law constant.
• The value of R is dependant on the
units of pressure and volume.
• The most common units used are pressure
in atm and volume in L. Therefore, the
constant is .0821atm*L/mol*K
• If pressure is in pa and volume in m3 then
8.314pa*m /mol*K is used.
3
A sample of Iodine gas is put into a bottle at 60c and
1.4atm of pressure. If the volume of the bottle is
500mL, how many grams of iodine is there?
Questions to ask:
What R will we use? What
conversions do we need to make 1st. What do we need to find
and then how do we get grams?
P = 1.4atm V = .5L T= 333K R= .0821 n= ?
(1.4atm)(.5L) = (n)(.0821)(333K) 
n = .03mols I2
(1.4atm)(.5L) = n
(.0821)(333K)
For Grams: .03mols I2 x 254g I2 = 7.62g I2