Chemistry Reference
In-Depth Information
This tells us that the less massive (helium) gas will diffuse twice as fast
as the more massive (methane) gas. This makes sense, of course, because if
two objects have the same kinetic energy, the more massive one must be
moving slower. Graham's Law applies this concept to gases.
Graham's Law states that “under equal conditions of temperature and
pressure, gases diffuse at a rate that is inversely proportional to the square
roots of their molecular masses.” If your teacher asks you which of a group
of gases will diffuse most quickly under equal conditions, the answer will
be the least massive gas. If the question involves the relative rate of diffu-
sion, you would solve the problem as shown in the comparison just pre-
sented. Let's try a typical example.
Example 1
Calculate the ratio of the velocity of neon atoms to the velocity of
oxygen molecules at the same temperature.
We will need the molecular mass of the oxygen, which is diatomic, and
the atomic mass of the neon, which exists as monatomic particles. We can
get this information from the periodic table.
Neon (Ne) = 20.2 u
Oxygen (O
2
) = 32.0 u
Now, we just use the formula that we derived in the comparison of two
different gases at equal temperatures and pressures. I always set it up so
that the more massive particle goes on the top of the fraction under the
radical sign.
M
V
32
.
0
u
O
Ne
2
1
.
584158
1
.
26
=
=
=
=
V
M
20
.
2
u
O
Ne
2
The less massive particles move faster, so we would say the neon dif-
fuses 1.26 times as fast as the oxygen, or the ratio of V
O
2
:V
Ne
= 1:1.26
We will do one more example. Although the wording may seem quite
different, it is still an example of a Graham's Law problem. Remember:
We find the masses of the elements involved by looking them up on the
periodic table.
