Chapter 5: 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
Expand to fit available Volume
(create a force on ALL sides of vessel : 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)
Standard atmospheric pressure is
:
76 cm Hg = 760 mm Hg = 760 torr = 1 atmosphere = 101.325 kPa = 1.01325 bar
(GIVEN ON EXAMS)
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 question.
T MUST
ALWAYS
BE IN KELVIN.
Boyles’ Law:
P
1
V
1
= constant
(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
= constant
(for constant n, P)
T
1
VOLUME is PROPORTIONAL to TEMPERATURE in KELVIN
EXTRAPOLATION 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!!
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentStandard 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
= constant
(for n constant)
It is
critical
that T is in K!!!
T
1
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?
Example: A sample of methane, CH
4
, occupies 2.60 x 10
2
mL at 32oC under a pressure of 0.500 atm.
At
what temperature would it occupy 5.00 x 10
2
mL under a pressure of 1.20 x 10
3
torr?
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '07
 Fakhreddine/Lyon
 Chemistry, Mole, pH

Click to edit the document details