Chapter 11:
Gases
Coach Kelsoe
Chemistry
Pages 360–389
Section 11–1:
Gases and Pressure
Coach Kelsoe
Chemistry
Pages 361–367
Section 11–1 Objectives
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Define pressure and relate it to force.
Describe how pressure is measured.
Convert units of pressure.
State the standard conditions of
temperature and pressure.
Pressure
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If you have a one-liter bottle of air,
there’s really no way to know how much
air you have in there unless you know
the conditions inside the bottle.
To describe a gas fully, you need to
state four measurable quantities:
• Volume
• Temperature
• Number of molecules
• Pressure
Pressure and Force
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If you blow air into a rubber balloon, it
will increase in size. The increase in
volume is caused by the collisions of
molecules of air with the inside walls of
the balloon. The collisions cause an
outward push, or force, against the
inside walls.
Pressure (P) is defined as the force per
area on a surface, or P = F/A.
Pressure and Force
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The SI unit for force is the newton,
abbreviated N. It is the force that will
increase the speed of a one kilogram
mass by one meter per second each
second it is applied.
At Earth’s surface, each kilogram of
mass exerts 9.8 N of force, due to
gravity.
Force on Earth can be found by
multiplying an object’s mass by 9.8.
Pressure and Force
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For example, if a person’s mass is 51
kg, then the amount of force they apply
on the Earth’s surface is 500 N. No
matter how a person stands, he or she
will exert 500 N of force on Earth.
However, a person can change the
amount of pressure they apply to the
earth’s surface by changing the area in
contact with the ground – by standing
on their tiptoes.
Pressure and Force
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Let’s say this 500 N person stands flatfooted. The area of his feet is equal to
100 cm2. The pressure he applies to
the ground is 5 N/cm2.
If this person stood on his tiptoes, and
the area of his tiptoes was 10 cm2, then
the pressure he applies to the ground is
50 N/cm2.
This is how the “bed of nails” trick
works, and the secret of karate.
Pressure and Force
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Gas molecules exert pressure on any
surface with which they collide.
The pressure exerted by a gas depends
on volume, temperature, and the
number of molecules present.
Even the atmosphere exerts pressure.
At sea level, the pressure is 10.1
N/cm2. This pressure can be thought of
as caused by the weight of the gases
that compose the atmosphere.
Pressure and Force
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To understand gas pressure and it’s
magnitude, let’s think about an empty
can. The can does contain a small
amount of air. Let’s say the can has a
total area of 1700 cm2. If the can is not
expanding or contracting, then the
pressure inside must be the same as
the outside. If Earth’s pressure is 10.1
N/cm2, then the can’s pressure is 10.1
N/cm2. You can find the force of the air
inside the can mathematically.
Measuring Pressure
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A barometer is a device used to
measure atmospheric pressure.
The first barometer was introduced by
Evangelista Torricelli in the early 17th
century.
Through Torricelli’s experiments, it is
known that at sea level at 0°C, the
height of mercury in a barometer will be
760 millimeters high.
Units of Pressure
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A number of different units are used to
measure pressure:
• Pascal
• mm Hg
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•Torr
•Atmosphere
Since atmospheric pressure is often
measured by a mercury barometer, a
common unit of measure is
millimeters of mercury (mm Hg).
Units of Measure
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In honor of Torricelli, 1 mm Hg is now
called 1 torr. The average atmospheric
pressure at sea level at 0°C is 760 torr.
Pressures are often measured in units of
atmospheres. One atmosphere of
pressure (atm) is defined as being
exactly equivalent to 760 torr.
Units of Measure
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In SI, pressure is expressed in derived units
called pascals, after Blaise Pascal, a French
mathematician and philosopher who studied
pressure during the 1600s.
One pascal (Pa) is defined as the pressure
exerted by a force of one newton acting on an
area of one square meter. It is often more
convenient to express pressure in terms of
kilopascals.
The standard atmosphere is equal to 1.01325
x 105 Pa, or 101.325 kPa.
Units of Measure
Unit
Pascal
Millimeter of
Mercury
Symbol
Definition/
Relationship
Pa
SI pressure unit;
1 Pa = 1N/m2
Pressure that
supports a 1 mm
mercury column
1 torr = 1 mm Hg
mm Hg
Torr
torr
Atmosphere
atm
Avg. atmospheric
pressure at sea
level; 1 atm = 760
torr, 101.325 kPa
Standard Temperature and Pressure
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To compare volumes of gases, it is
necessary to know the temperature and
pressure at which the volumes are
measured.
For purposes of comparison, scientists
have agreed on standard conditions of
exactly 1 atm pressure and 0°C. These
conditions are called standard
temperature and pressure and
commonly abbreviated as STP.
Sample Problem 10-1
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The average atmospheric pressure in
Denver, Colorado, is 0.830 atm.
Express this pressure (a) in mm Hg
and (b) in kPa.
• Given: P of atmosphere = 0.830 atm, 760
mm Hg = 1 atm, 101.325 kPa = 1 atm
a) 0.830 atm x 760 mm Hg/atm = 631 mm
Hg
b) 0.830 atm x 101.325 kPa/atm = 84.1 kPa
Vocabulary
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Atmosphere of pressure
Barometer
Millimeters of mercury
Newton
Pascal
Pressure
Standard temperature and pressure
Torr
Conversion Factors
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1
1
1
1
1
1
torr = 0.1333 kPa
kPa = 7.5 torr
atm = 101.325 kPa
kPa = 0.00987 atm
atm = 760 torr
torr = 0.001316 atm