Gas Laws
Chapter 13
Elements that exist as gases at 250C and 1 atmosphere
Observed GAS physical properties
 Gases
have mass
 Gases can compress (springy)
 Gases FILL containers completely
 Gases diffuse through each other (bad
smells).
 Gases exert pressure
 Pressure depends on temperature.
 These properties led to the Kinetic
molecular theory (KMT)
The KMT says that gases….….
 Consist
of small particles having mass
 Distance between particles large (particle
size close to 0)
 Particles always move at random speeds
in random directions.
 Perfectly elastic collisions (no energy lost)
 Average KE of particles proportional to T.
 Forces between particles is zero.
Measuring Gases
 n,
moles of gas =
Mass___
Molar Mass
• Volume in L . (1000cm3 = 1L)
• Temperature, K . (K = °C +
273.15)
• Pressure , Pascals (Pa)
10 miles
4 miles
Sea level
0.2 atm
0.5 atm
1 atm
 Above
Atmospheric Pressure
every square inch of the earth’s
surface is about 15 Lbs of air. It is like
being at the bottom of a gas sea.
 Higher up, ex.Denver there is ___
pressure
 Think of aircraft altimeters,
 Ears, Cooking, weather,Sports.
 Barometer measures atmospheric
pressure.
Pressure = Force
Area
Units of Pressure
1 pascal (Pa) = 1 N/m2
1 atm =760mmHg=760 torr
1 atm = 101,325 Pa
Barometer
Closed
End
Open
ended
Using a manometer

Manometer measures pressure differences.
We are only using open-ended ones in the
problems (P13)
 Pressure in connected container (mmHg) =
height difference + atmospheric pressure.
The height diff. is (+) when the open end is higher
than the closed end.
The height diff. is (-) when the open end is ___er
than the closed end.
ALWAYS DRAW A PICTURE FOR MANOMETER
PROBLEMS!!!!!!!!
You must be able to convert
between units of pressure……
 Use
the following conversion factors:
 1atm = 101,325 Pa
 1atm = 760mm Hg
 1 atm = 14.70 Lb/in2
 1 bar = 100,000 Pa
 Let’s do some problems on the
board…..
 How
many mmHg is 3.5atm?
 Convert 35psi to atm.
 How many Pa is 6.4 atm?
 Convert 5000Pa to atm.
 The next ones must got to atm first and
then to the desired unit
 How many psi is 50.0 Pa?
 How many mmHg is 200,000Pa?
 How many psi is 2500Pa?
Section 13.3
What happens the volume of a gas when the
pressure increases?
 This is described by Boyles Law :
 “At a constant temperature, the volume of a gas
is inversely proportional to the pressure”
 This is expressed by

P1 V1 = P 2 V2
P is pressure, V is Volume (L), 1 means initial, 2
means final
Some examples of Boyles Law
Bag of chips taken from sea level to Denver.
What happens the bag?
 Take a helium balloon from top of Sears tower
to street level. What happens?
 What happens your eardrum as you go up in a
plane?

Charles Law

“At a constant pressure, the volume of a gas is
proportional to the temperature”
 V1T2
= V2T1
Examples:
 Hot air balloon
 Internal combustion engine
 Explosion
 What would happen if you took a balloon and
put it in the freezer?

Pop Test
Please take out FIVE blank sheets of
paper and your calculators.
 Solve the following 20 problems, showing
all work using the G.U.E.S.S. method.
 1) Find the new Volume of a hedgehog
travelling at 0.5m/s with a body
temperature of 5K going north. The mass
of the hedgehog is 1.5kg.

2) What date is it today?
 Got Ya !!!!!

Dalton’s Law of Partial Pressures
 If
there is more than one gas present, the
total Pressure = sum of pressures of each
gas
Pa + Pb + …….. + Pz
 Let’s do a simple example.
 Pt =
Avogadro’s Law
 The
volume of a gas at STP is
proportional to the number of moles
present.
V=kn
V
, Volume in L
 k , k molar constant (22.4L/mol)
 n , moles
Ideal gas Equation
 Most
gases act as ideal gases except
when close to 0K and/or under extreme
pressure.
 Boyles Law, Charles Law and Avogadro’s
Law can be combined into an all-purpose
Law allowing any “gas problem” to be
solved (kind of like Beano) called the Ideal
gas Law
Ideal Gas Law
PV = nRT
n , moles of gas
 V, Volume (L)
 P , pressure (atm)
 T , temperature (K)
 R , Ideal gas constant (0.0821 atm-L/mol-K)
 For problems with initial and final
conditions, use…..
 P1V1 / T1 = P2V2 / T2
There are lots of examples:
What happens the temperature and pressure of
CO2 gas in a paintball gun when it fires?
 What happens the temperature and pressure of
wind as it blows up a slope?
 What happens the temperature and pressure of
wind as it blows down a slope?
 What happens the temperature and pressure of
air in a pump?
 The above example explains how a diesel
engine works. Can you think of any other
examples?

Some things to know about gases – CH13.5
Lifting power of gases:
 All objects in a gas have a buoyant force on
them. This force is in the upward direction.
It depends on the size of the object.
 Weight is a force in the downward direction.
 A balloon with a light gas in it has a smaller
weight than one with a heavier gas. Can you
think of some light gases?
 If buoyant force on the balloon > weight,
balloon rises. If buoyant force<weight, it
descends.

A way to make a gas less dense is by heating.
 A more expensive way is to use He gas or H2.
 What would be an advantage of each?

The ozone hole – causes/effects
Ozone (O3) is formed by the action of sunlight
on O2 in the upper stratosphere.
 Ozone filters out most harmful U.V. rays.
 Some manmade substances –
chlorofluorocarbons (CFC’s) in particular cause
this reaction to be reversed, destroying Ozone.
 CFC’s are used in refrigerators, air conditioners
and some plastic foam products.
 An ozone hole developed over the Arctic and
Antarctic with reduced levels of ozone
worldwide.
 Since most CFC production has been banned
since 1980, the problem has improved
somewhat.
