Solution

urms



urms 3 .6 2  10 ^6 m^2 /s^2  1.90 103 m/s (about 4250 mph)

You should now work Exercise 86.

DIFFUSION AND EFFUSION OF GASES

Because gas molecules are in constant, rapid, random motion, they diffuse quickly

throughout any container (Figure 12-13). For example, if hydrogen sulfide (the smell of

rotten eggs) is released in a large room, the odor can eventually be detected throughout

the room. If a mixture of gases is placed in a container with thin porous walls, the mole-

cules effuse through the walls. Because they move faster, lighter gas molecules effuse

through the tiny openings of porous materials faster than heavier molecules (Figure

12-14).

Although they are the most abundant elements in the universe, hydrogen and helium

occur as gases only in trace amounts in our atmosphere. This is due to the high average

molecular speeds resulting from their low molecular weights. At temperatures in our

atmosphere, these molecules reach speeds exceeding the escape velocity required for them

to break out of the earth’s gravitational pull and diffuse into interplanetary space. Thus,

most of the gaseous hydrogen and helium that were probably present in large concentra-

tions in the earth’s early atmosphere have long since diffused away. The same is true for

the abundance of these gases on other small planets in our solar system, especially those

with higher average temperatures than ours (Mercury and Venus). The Mariner 10 space-

craft in 1974 revealed measurable amounts of He in the atmosphere of Mercury; the source

of this helium is unknown. Massive bodies such as stars (including our own sun) are mainly

H and He.

12-14

3 8.314 

m

k

o

g

l



K

m



2

s^2

293 K



2.016 

m

g

ol



10

1

0

k

0

g

g



3 RT



M

Scientists use the word “effusion” to
describe the escape of a gas through a
tiny hole, and the word “diffusion” to
describe movement of a gas into a
space or the mixing of one gas with
another. The distinction made by
scientists is somewhat sharper than
that usually found in a dictionary.

470 CHAPTER 12: Gases and the Kinetic–Molecular Theory

Figure 12-13 A representation of diffusion of gases. The space between the molecules

allows for ease of mixing one gas with another. Collisions of molecules with the walls of the

container are responsible for the pressure of the gas.

O 2 + N 2

( – atm each)^12

Equal

volumes

Stopcock

closed

O 2 (1 atm) N 2 (1 atm)

(a)

Stopcock

open

(b)

O 2 + N 2

( – atm each)^12

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 12.12, Applications of the
Kinetic–Molecular Theory.