Chemistry Reference
In-Depth Information
all directions. What factors determine the amount of time that elapses from
when she enters the room and when you smell her perfume? A study of
Graham's Law will allow you to answer this and other questions.
The first factor that will affect the rate of diffusion will be the tempera-
ture of the molecules. Remember that we defined temperature as the av-
erage kinetic energy of the molecules of a substance. Remember, also, that
the formula for kinetic energy is:
K.E. = 1/2 MV
2
where M is the mass and V is the velocity of the object
When we heat a substance, its temperature increases as the speed of its
molecules increases. Why would a woman apply perfume to her neck and
wrists? Because those are particularly hot areas of the body, which will
heat the perfume faster and allow the molecules of the perfume to leave
her body faster and travel faster.
How does the mass of the molecules affect the rate of diffusion? Let's
compare the molecules of two different gases, say methane (CH
4
) and he-
lium (He), at equal conditions of temperature and pressure.
Comparison of 2 Different Gases at Equal Temperatures and Pressures
CH
4
He
If they are at the same temperature, then
1/2 M
1
V
1
2
= 1/2 M
2
V
2
2
If we multiply both sides by 2, then
M
1
V
1
2
= M
2
V
2
2
2
1
V
M
2
Now, we rearrange the equation
=
2
2
M
V
1
V
=
M
Taking the square root of both sides, we get
1
2
V
M
2
1
Now, the molecular mass of methane is 16.0 u and the mass of
helium is 4.00 u. Filling
t
hese values in for M
1
and M
2
, we get:
V
16
u
1
4
2
=
=
=
V
4
u
2
Stating as a ratio of V
1
to V
2
we get V
1
:V
2
= 2:1
