Transcript Slide 1

Chapter 9
Gases
Gases and Gas Pressure
Gas mixtures are homogeneous and compressible.
Atmospheric Pressure
Atmospheric pressure
is the pressure exerted by a
column of air from the top
of the atmosphere to the
surface of the Earth.
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Pressure Imbalance in Ear
If there is a difference
in pressure across
the eardrum membrane,
the membrane will be
pushed out – what we
commonly call a
“popped eardrum.”
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Gases and Gas Pressure
A barometer
• measures the pressure exerted
by the gases in the atmosphere.
• indicates atmospheric pressure
as the height in mm of the
mercury column.
Gas Pressure
Gas pressure
• is a force acting on a specific area.
Pressure (P) = force
area
• has units of atm, mmHg, torr, lb/in.2, and kilopascals(kPa).
1 atm
1 atm
=
=
760 mm Hg (exact)
760 torr
1 atm
=
14.7 lb/in.2
1 atm
=
101 325 Pa
1 atm
=
101.325 kPa
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Gases and Gas Pressure
Monometer
Pgas < Patm
Pgas = Patm - PHg
Pgas > Patm
Pgas = Patm + PHg
The Gas Laws
Ideal Gas: A gas whose behavior follows the gas laws exactly.
The physical properties of a gas can be defined by four variables:
P
pressure
T
temperature (calculation must be in Kelvin)
V
volume
n
number of moles
The Gas Laws
Boyle’s Law
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V a
P
(constant n
and T)
PinitialVinitial = PfinalVfinal
The Gas Laws
Boyle’s Law
V a
1
P
(constant n and T)
PV Constant in Boyle’s Law
In Boyle’s law, the product P x V is constant as long
as T and n do not change.
P1V1 = 8.0 atm x 2.0 L = 16 atm L
P2V2 = 4.0 atm x 4.0 L = 16 atm L
P3V3 = 2.0 atm x 8.0 L = 16 atm L
Boyle’s law can be stated as
P1V1 =
P2V2
(T, n constant)
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Boyle’s Law and Diving
 since water is denser than
air, for each 10 m you dive
below the surface, the
pressure on your lungs
increases 1 atm
 at 20 m the total pressure
is 3 atm
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if your tank
contained air at 1
atm pressure you
would not be able
to inhale it into
your lungs
Boyles’ Law and Breathing
During an inhalation,
• the lungs expand.
• the pressure in the
lungs decreases.
• air flows towards the
lower pressure in the
lungs.
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Boyles’ Law and Breathing
During an exhalation,
• lung volume
decreases.
• pressure within the
lungs increases.
• air flows from the
higher pressure in the
lungs to the outside.
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Example
If a sample of helium gas has a volume of 120 mL
and a pressure of 850 mmHg, what is the new
volume if the pressure is changed to 425 mmHg?
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The Gas Laws
Va T
In Charles’s Law,
• the Kelvin temperature
of a gas is directly related
to the volume.
• P and n are constant.
• when the temperature of
a gas increases, its
volume increases.
The Gas Laws
Va T
Charles’s Law
(constant n and P)
The Gas Laws
Charles’s Law
Va T
(constant n and P)
V
T
Vinitial
Tinitial
=k
=
Vfinal
Tfinal
Examples
A sample of oxygen gas has a volume of 420 mL at a
temperature of 18 °C. At what temperature (in °C)
will the volume of the oxygen be 640 mL (P and n
constant)?
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The Gas Laws
Avogadro’s Law
Va n
(constant T and P)
V
n
Vinitial
ninitial
=k
=
Vfinal
nfinal
Examples
If 0.75 mole of helium gas occupies a volume of 1.5 L,
what volume will 1.2 moles of helium occupy at the
same temperature and pressure?
The Ideal Gas Law
Summary
If the systems is disturbed by one of the four variables: O, T, n then co
the following changes
Boyle’s Law:
Charles’ Law:
Avogadro’s Law:
PinitialVinitial = PfinalVfinal
Vinitial
=
Vfinal
Tinitial
Tfinal
Vinitial
Vfinal
ninitial
=
nfinal
Combine Gas Law


is an expression obtained by mathematically
combining Boyle’s and Charles’ law
P1V1 = P2V2
@ constant n
T1
T2
can predict P, V or T when condition is changed
Examples
A gas has a volume of 675 mL at 35 °C and 0.850
atm pressure. What is the volume (mL) of the
gas at -95 °C and a pressure of 802 mmHg? (n
constant)
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The Ideal Gas Law
Ideal Gas Law:
PV = nRT
R is the gas constant and is the same for all gases.
R = 0.082058
L atm
K mol
Standard Temperature and Pressure
(STP) for Gases
T = 0 °C (273.15 K)
P = 1 atm
Examples
 What volume is occupied by 25.7 g of carbon dioxide
gas at 25.0oC and 371 torr?
 A 0.250 mol sample of argon gas has a volume of 9.00L
at a pressure of 875 mmHg. What is the temperature
(in oC) of the gas?
 Determine the molar mass of a gas with a density of
1.905 g/L at 80.0oC
The Ideal Gas Law
What is the volume of 1 mol of gas at STP?
(1 mol)
V=
nRT
P
0.08206
L atm
(273.15 K)
K mol
=
= 22.41 L
(1 atm)
Molar Volume
At standard temperature and pressure (STP), 1
mole of a gas occupies a volume of 22.4 L, which is
called its molar volume.
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Example
A. What is the volume at STP of 4.00 g of CH4?
1) 5.60 L
2) 11.2 L
3) 44.8 L
B. How many g of He are present in 8.00 L of gas at
STP?
1) 25.6 g
2) 0.357 g
3) 1.43 g
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