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Chapter 9 - GASES
ALG
• Open your ALG page 9-5:
9.2 Pressure, density,
and the mass of
particles
• Work in groups and answer questions
• 9.2.1 Page 9-5 - Whiteboard
• 9.2.2 Page 9-5
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PRESSURE
• As air particles move
randomly in space,
they eventually collide
with the solid surfaces
of any objects in that
space. In each of
these collisions, the
particle exerts an
impulsive force on the
object—like a tennis
ball hitting a practice
© 2014 Pearson Education, Inc.
wall.
Pressure
PRESSURE
• An extremely large number of gas particles
collide each second with the surface.
• Forces are so small and so frequent, that this
force can be assumed to be constant.
• Force depends on the area of the surface.
• Instead of using a force to describe gas processes,
we use the Physical quantity PRESSURE
Measuring pressure
• The SI unit of pressure is the pascal (Pa),
where 1 Pa = 1 N/m2.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
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ATMOSPHERIC PRESSURE
Gauge pressure
• Measurements show that the pressure of the
atmospheric air at sea level is on average:
𝑃𝑎𝑡𝑚 =
1𝑥105 N
𝑚2
𝑃𝑎𝑡𝑚 =1𝑥105 𝑃𝑎
Atmosphere: 1.0 atm = 𝑃𝑎𝑡𝑚 =105 𝑃𝑎
© 2014 Pearson Education, Inc.
• When you use a tire gauge to measure the air
pressure in a car tire, you are comparing the
pressure inside the tire to the pressure of the
atmosphere outside the tire.
© 2014 Pearson Education, Inc.
WHITEBOARD
Quantitative Exercise 9.1
Estimate the total force that air exerts on the
front side of your body, assuming that the
pressure of the atmosphere is constant.
Density
• Mass describe solid objects that have real
boundaries.
• For gases, a much more useful physical
quantity than mass of the individual particles
is the mass of one unit of volume—density.
1.Sketch and translate.
2.Simplify and diagram.
3.Represent mathematically.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Tip
WHITEBOARD
Quantitative Exercise 9.2
Estimate the density of a person.
1.Simplify and diagram.
2.Represent mathematically.
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Mass and size of particles
Avogadro's number and the mole
• In 1811, an Italian scientist named Avogadro
proposed that equal volumes of different
types of gas, when at the same temperature
and pressure, contain the same number of gas
particles.
• The mass in grams of any substance that has
exactly Avogadro's number of particles is
equal to the atomic mass of that substance.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Mass and size of particles
• Presently, scientist use Avogadro’s number (NA)
to indicate the number of atoms or molecules
present in 22.4 L (22. 4x103 cm3) of any gas at 0°C
and standard atmospheric pressure.
© 2014 Pearson Education, Inc.
WHITEBOARD
Quantitative Exercise 9.3
Determine the average separation between
nearby gas particles in the air, and compare it
with the size of the particles themselves.
The average diameter of a single particle of air
is 3x10-8 cm
1.Sketch and translate.
2.Simplify and diagram.
3.Represent mathematically.
© 2014 Pearson Education, Inc.
ALG
• Open your ALG page 9-5:
• Work in groups and answer questions
• 9.2.3 Page 9-5
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