Section 9.2
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Pressure: the force per unit area
Kinetic Molecular Theory of Gases states that a gas
consists of particles in constant, random, straightline motion.
The pressure of a gas is the force exerted by the
molecules as they collide with objects in their path
(walls of the container)
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quantitative measurements on gases were
first made by the English chemist, Robert
Boyle (1627 - 1691).
Boyle used two instruments to measure
pressure: the manometer, which measures
differences in pressure, and the barometer,
which measures the total pressure of the
atmosphere
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A barometer uses the height of a column of
mercury to measure gas pressure
The mercury is pushed up the tube from the dish
until the pressure at the bottom of the tube (due to
the mass of the mercury) is balanced by the
atmospheric pressure
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units of pressure were originally based on the
length of the column of liquid supported in a
manometer or barometer.
the most common of these units was the mm Hg
(1 mmHg = 1 torr).
however, the modern SI unit of pressure is the
pascal, Pa (1 Pa = 1 N/m2)
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force per unit area exerted by air on all objects
at sea level, it is 101.325 kPa (1 kPa = 1000 Pa)
scientists used this value to define one standard
atmosphere (1 atm)
therefore, 1atm = 101.325 kPa
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Scientists have agreed to use a set of
standard conditions for reporting properties
of gases and other substances, SATP.
Standard Ambient Temperature and Pressure
(SATP) is 25° C and 100 kPa.
Previous conditions used were referred to as
STP (standard temperature and pressure)
STP is 0° C and 101.325 kPa
Unit Name
Unit Symbol
Definition/Conversion
pascal
Pa
1 Pa = 1 N/m2
atmosphere
atm
1 atm = 101.325 kPa
millimeters of mercury mmHg
760 mmHg = 1 atm = 101.325 kPa
torr
1 torr = 1 mmHg
torr
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Boyle used the manometer and barometer to
study the pressures and volumes of different
samples of different gases. The results of his
studies can be summarized in a simple
statement which has come to be known as
Boyle's law:
At any constant temperature, the product of
the pressure and the volume of any size
sample of any gas is a constant.
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For a particular sample of any gas, Boyle's law
can be shown graphically as done in the
Figure below. It is more common to express it
mathematically as P1V1 =P2V2
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This means that the pressure and the volume
vary inversely; as the pressure increases, the
volume of the gas must decrease and vice
versa.
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A sample of gas occupies a volume of 47.3
cm3 at 25°C with a pressure of 30 mm of
mercury. If the pressure is increased to 75
mm of mercury, what will the volume be at
the new pressure?
G
P1 = 30 mmHg
V1 = 47.3 cm3
P2 = 75 mmHg
S V2 = (30 mmHg x 47.3 cm3)
75 mmHg
V2 = 18.9 cm3
R
V2 = ?
A
P1V1 = P2V2
V2 = P1V1
P2
P Therefore, when the pressure
increased from 47.3 mmHg
to 75 mmHg, the volume
decreased from 47.5 cm3 to
18.9 cm3
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A student is trying to compress the gas in a
cylinder that has an initial volume of 1000
cm3 and a pressure of 100 kPa. The student
decreases the volume by 500 cm3. What is the
pressure of the gas in the cylinder?
G
P1 = 100 kPa
V1 = 1000 cm3
V2 = 500 cm3
S P2 = (100 kPa x1 000 cm3)
500 cm3
P2 = 200 kPa
R
P2 = ?
A
P1V1 = P2V2
P2= P1V1
V2
P Therefore, when the student
compressed the cylinder to
half of its original volume,
the pressure doubled to 200
kPa.
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Read Section 9.2 (pg. 423 – 429)
Questions: page 425 # 1 – 4, page 428 # 5-9.