Chapter 4: PROPERTIES OF GASES;
THE GAS LAWS
GASES:
molecules very far apart.
LIQUIDS, SOLIDS
(CONDENSED PHASES): molecules touch
gas density << liquid / solid density
Properties of Gases
Expands to fit available Volume
(creates a force on ALL sides of vessel described by a pressure measurement )
Compressible.
Volume & Pressure vary greatly with Temperature.
Pressure.
Definition:
force per unit area.
SI Unit: N/m
2
= 1kg.m
1
.s
2
= 1 Pascal (Pa)
Other Units:
lb/in
2
= 1 pound per square inch = 1 PSI, cm or mm Hg,
torr,
atmospheres
Atmospheric Pressure:
Pressure exerted by Earth’s atmosphere on a given day in a given location.
Measured
directly.
Standard atmospheric pressure is
:
76 cm Hg = 760 mm Hg = 760 torr = 1 atmosphere = 101.325 kPa = 1 bar
Measuring Presure
directly
 use a
barometer
indirectly
 use a manometer
Units are the same in each case
How a Barometer Works
Gravity pulls down on Hg in column; atmospheric pressure pushes on pool of Hg to force it up the tube;
forces balance; supports column of Hg
P =
ρ
g h
If atmospheric pressure
drops
, column will
shorten
.
How a Manometer Works
U tube with Hg.
Height Difference
gives
pressure
difference
. Use barometer reading to get
actual
pressure
THE GAS LAWS
Use the following parameters: Pressure, P ; Temperature, T; Volume, V;
Number of moles of Gas, n
P,V do not always have to be in SI units AS LONG AS you are careful to read the Q.
T MUST
ALWAYS
BE IN KELVIN.
Boyles’ Law:
P
1
V
1
=P
2
V
2
(for constant n, T)
PRESSURE is INVERSELY RELATED to VOLUME:
Example: At 25
o
C a sample of He has a volume of 4.00 x 10
2
mL under a
pressure of 7.60 x 10
2
torr.
What volume would it occupy under a pressure of 2.00 atm at the same T?
Charles’ Law and The Absolute Temperature Scale:
V
1
= V
2
(for constant n, P)
T
1
T
2
VOLUME is PROPORTIONAL to TEMPERATURE in KELVIN
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View Full DocumentEXTRAPOLATION of VT plot backwards gives ABSOLUTE ZERO:
ZERO KELVIN.
0 ° C is where water freezes 
not
absolute
zero.
0K = absolute zero, all molecular motion ceases and ideal gas* volume tends to zero
0° C = 273.15 K
*real gases liquefy before reaching zero K
Example: A sample of hydrogen, H
2
, occupies 1.00 x 10
2
mL at 25.0
o
C and 1.00 atm.
What volume would
it occupy at 50.0
o
C under the same pressure? It is CRITICAL to convert ANY temperature to KELVIN!!
Standard Temperature and Pressure
A convenient reference point.
Symbol:
STP.
Standard P
≡
1.00000 atm or 101.3 kPa
Standard T
≡
273.15 K or 0.00
o
C
LEARN THIS!!
Combined Gas Law:
Combining Boyle’s and Charles’ Laws gives :
P
1
V
1
= P
2
V
2
(for n constant)
T
1
T
2
REMEMBER: it is
critical
to make sure that T is in K, and V and P are given in the same units
before and after the change.
Example: If we have a 2.5L of a gas at 25°C and 1.5atm, and we increase the temperature by 100 °C and
find the pressure is now 800 torr, what is the new volume?
Example:
A sample of nitrogen gas, N
2
, occupies 7.50 x 10
2
mL at 75.00C under a pressure of 8.10 x 10
2
torr.
What volume would it occupy at STP?
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 Spring '08
 FATIMAFAKHREDDINE
 Mole, pH

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