Diffusion/Effusion…
…no need for confusion
Why did the NH3 in the previous demo diffuse
faster than the HCL?
Because each molecule is less
massive and, therefore, FASTER
Let’s see how diffusion/effusion
works…
Here’s one application of effusion
Many small holes (10-6
inch!) allow UF6
molecules with lighter
mass isotopes of
uranium pass through
more frequently…
…so a greater
amount of the
lighter isotopecontaining UF6
exits here
Simply put…

Light stink diffuses faster than
heavy stink
• of course, a gas doesn’t have to have an odor
for it to diffuse
now, a little mathematical
understanding of diffusion/effusion…
Graham’s Law of Effusion
(works with diffusion as well)

Any moving object’s kinetic energy (KE)
is expressed as:
1
2
KE  mv
2

m = mass
v = velocity
So, when comparing any two gases (gas
“A” and gas “B”), their KE’s can be
expressed as:
1
2
KEA  mA v A
2
1
2
and KEB  mB vB
2

Recall that, at any given temperature,
the average KE of any gas is the same.
Therefore, if you compare any two
gases (gas “A” and gas “B”) at the
same temperature, it follows that:
KEA  KEB

Which can also be expressed as:
1
1
2
2
mA v A 
mB vB
2
2
The previous equation can be rearranged to
yield Graham’s Law of Effusion:
vA

vB
mB
mA
(also applies to diffusion)
Graham’s Law of Effusion
The rate of effusion (and diffusion) of a
gas is inversely proportional to the
square root of the gas’s molar mass