Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
119
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 12
Example Problem 12-1
Finding the Ratio of Diffusion Rates
The molar mass of helium is 4.00 g/mol; the molar mass of air is
29.0 g/mol. What is the ratio of their diffusion rates? Which gas
diffuses faster?
ᎏ
R
R
a
a
t
t
e
e
A
H
i
e
r
ᎏ ϭ
Ί
ᎏ
m
m
o
o
l
l
a
a
r
r
๶
m
m
a
a
s
s
s
s
A
H
๶
i
e
r
ᎏ
๶
ϭ Ί ᎏ
2
4
9
.0
.0
0
g
g
๶
/
/
m
m
o
o
l
l
ᎏ ๶ ϭ ͙7.25 ෆ ϭ 2.69
The ratio of the diffusion rates is 2.69. Helium diffuses about
2.7 times faster than air does.
Practice Problems
1. Calculate the ratio of diffusion rates for neon and helium.
Which gas diffuses faster? About how much faster?
2. Calculate the ratio of diffusion rates for ammonia (NH 3 ) and
carbon dioxide (CO 2 ). Which gas diffuses more rapidly?
3. What is the ratio of diffusion rates for argon and radon? Which
gas diffuses more rapidly?
4. Tetrafluoroethylene (C 2 F 4 ) effuses at a rate of 4.6 ϫ 10 Ϫ6
moles per hour. An unknown gas effuses at a rate of 6.5 ϫ 10 Ϫ6
moles per hour under identical conditions. What is the molar
mass of the unknown gas?
Gas pressure When gas particles collide with the walls of their
container, they exert pressure on the walls. Pressure is force per unit
area. The pressure exerted by the particles in the atmosphere that
surrounds Earth is called atmospheric pressure, or air pressure. Air
pressure varies at different locations on Earth. At Earth’s surface,
air pressure is approximately equal to the pressure exerted by a
1-kilogram mass on a square centimeter. Air pressure at higher altitudes, such as on a mountaintop, is slightly lower than air pressure
at sea level.
Air pressure is measured using a barometer. A barometer consists of a thin tube closed on one end and filled with mercury. The
▲