# Chapter 9

```John E. McMurry • Robert C. Fay
C H E M I
S T R Y
Chapter 9
Gases: Their Properties and Behavior
Lecture Notes
Alan D. Earhart
Southeast Community College • Lincoln, NE
Gases and Gas Pressure
Gas mixtures are homogeneous and compressible.
Chapter
9/2
Gases and Gas Pressure
Pressure:
Force
Unit area
Chapter
9/3
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.”
Tro, Chemistry: A Molecular
Approach
4
Gases and Gas Pressure
Barometer
Units
Pa
torr
mm Hg
atm
bar
Chapter
9/5
Gases and Gas Pressure
Conversions
1 atm = 760 mm Hg
(exact)
1 torr = 1 mm Hg
(exact)
1 bar = 1 x 105 Pa
(exact)
1 atm = 101 325 Pa
Chapter
9/6
Gases and Gas Pressure
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
Chapter
9/8
The Gas Laws
Boyle’s Law
1
V a
P
(constant n
and T)
The Gas Laws
V a
1
P
Boyle’s Law
(constant n and T)
The Gas Laws
V a
Boyle’s Law
1
P
(constant n and T)
PV = k
PinitialVinitial = PfinalVfinal
Chapter
9/11
Boyle’s Law and Diving
 since water is denser than
air, for each 10 m you dive
below the surface, the
increases 1 atm
 at 20 m the total pressure
is 3 atm
Tro, Chemistry: A Molecular
Approach
12
contained air at 1
atm pressure you
would not be able
to inhale it into
Examples
 Calculate the volume of a sample of a gas at 5.75 atm if
it occupies 5.14 L at 2.49 atm. (Assume constant
temperature)
The Gas Laws
Charles’ Law
Va T
(constant n and P)
V
=k
T
Vinitial
Tinitial
=
Vfinal
Tfinal
Chapter
9/14
The Gas Laws
Charles’s Law
Va T
(constant n and P)
The Gas Laws
Charles’s Law
Va T
(constant n and P)
V
T
Vinitial
Tinitial
=k
=
Vfinal
Tfinal
Chapter
9/16
Examples
 A sample of argon gas that originally occupied 14.6 L at
25.0oC was heated to 50.0oC at constant pressure. What is
its new volume?
The Gas Laws
Va n
(constant T and P)
V
n
Vinitial
ninitial
=k
=
Vfinal
nfinal
Chapter
9/18
Examples
 Consider two samples of nitrogen gas (composed N2
molecules). Sample 1 contains 1.5 mol of N2 and has a
volume of 36.7 L at 25.0oC and 1 atm. Sample 2 has a
volume of 16.0 L at 25.0oC and 1.0 atm. Calculate the
number moles of N2 in sample 2
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:
PinitialVinitial = PfinalVfinal
Vinitial
=
Vfinal
Tinitial
Tfinal
Vinitial
Vfinal
=
ninitial
nfinal
Chapter
9/20
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
 Suppose we have a 0.240 mol sample of ammonia gas at
25.0oC with a volume of 3.5 L at a pressure of 1.68 atm.
The gas compressed to a volume of 1.35 L at 25.0oC. Use
the combined gas law to calculate the final pressure.
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
Chapter
9/23
Examples
 What volume is occupied by 25.7 g of carbon dioxide
gas at 25oC 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?
The Ideal Gas Law
Is there a mathematical relationship between
P, V, n, and T for an ideal gas?
Chapter
9/25
The Ideal Gas Law
What is the volume of 1 mol of gas at STP?
(1 mol)
V=
nRT
0.08206
L atm
(273.15 K)
K mol
=
P
= 22.41 L
(1 atm)
Chapter 9/26
examples
 A sample of nitrogen gas has a volume of 1.75 L at STP.
How many moles of N2 are present?
Example
 Sodium peroxide (Na2O2) is used to remove carbon dioxide
from (and add oxygen to) the air supply in spacecrafts. It
works by reacting with CO2 in the air to produce sodium
carbonate (Na2CO3) and O2.
2 Na2O2(s) + 2 CO2(g)  2 Na2CO3(s) + O2(g)
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