Chemistry Reference
In-Depth Information
2.3.2
Weight Distribution,
M
w
If we had recorded the weight of each species in the sample, rather than the number
of molecules of each size, the array of data would be a weight distribution. The situa-
tion corresponds to that described for a number distribution.
Figure 2.3
depicts a sim-
ple integral weight distribution, normalized by recording fractions of the total weight
rather than actual weights of the different species.
The integral (cumulative) weight fraction W(M) is given by
X
W
ð
M
Þ 5
w
i
(2-12)
i
and is equal to the weight fraction of the sample with molecular weight not greater
than M
i
. A plot of w
i
against M
i
yields a differential weight distribution curve, as in
Fig. 2.4
. As in the case of the number distribution, if W(M) is normalized, the scale of
the ordinate in this figure goes from 0 to 1 and the area under the curve equals unity.
1.0
0.8
0.6
0.4
0.2
0
Molecular weight,
M
i
FIGURE 2.3
A normalized integral weight distribution curve.
Molecular weight,
M
i
FIGURE 2.4
A normalized differential weight distribution curve.
