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Gases and Pressure
Kinetic-molecular theory is based on the idea that particles of matter are always in motion
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The pressure increase is caused by the
increase in the number of collisions of molecules of air with the inside walls of the tire
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Gas molecules exert pressure on any surface with which they collide
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Pressure (P) is defined
as the force per unit area on a surface
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It is the force that will increase the speed of a one-kilogram mass by one meter
per second each second that the force is applied
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8 m/s
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A mass of 51 kg exerts a force of 500 N (51 kg x 9
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No
matter how the dancer stands, she exerts that much force against the floor
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The same force applied to a smaller area results in a greater pressure
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The atmospheric pressure at sea level is about equal to the weight of a 1
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1 N/cm
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The atmosphere contains about 78% nitrogen, 21% oxygen, and 1% other
gases, including argon and carbon dioxide
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Your ears “pop” when you fly in an airplane because the density—and therefore the pressure—of the air is lower at higher
altitudes
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Measuring Pressure
A barometer is a device used to measure atmospheric pressure
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Torricelli wondered why water pumps could raise water to a maximum height of only
about 34 feet
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Therefore, he reasoned that liquid mercury, about 14 times dense as water, could be raised only 1/14 as high as water
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In every case, the mercury dropped to a height
of about 760mm
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The pressure is measure in terms of the mercury column’s height in the barometer tube
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A device called a manometer can be used to measure the
pressure of an enclosed gas sample
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Units of Pressure
A number of different units are used to measure pressure
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Thus, the common unit of pressure is
millimeters of mercury, symbolized mm Hg
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The average atmospheric pressure at sea level at 0 C is 760 mm Hg
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One atmosphere of pressure (atm) is defined as being exactly
equivalent to 760 mm Hg
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This unit is named for Blaise Pascal
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In many cases, it is
more convenient to express pressure in kilopascals (kPa)
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013 25 x 10 Pa,
or 101
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Standard Temperature and
Pressure
To compare volumes of gases, one must
know the temperature and pressure at which
the volumes are measured
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These conditions are
called standard temperature and pressure
and are commonly abbreviated STP
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013 25 x 10
Pa
= 101
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892 86 x 10
Pa
1 atm= 14
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The pressure
of each gas in a mixture is called the partial
pressure of that gas
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The law is true regardless of the number of different gases that are present
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PT = P1 + P2 + P3 + …

PT is the total pressure of the mixture
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