If a polythene sample contains 2 monodisperse fractions in the ratio 2...
Calculating Weight Average Molecular Weight of Polythene Sample
Understanding the Problem
In this problem, we are given a polythene sample that contains two monodisperse fractions in the ratio 2:3. The degree of polymerization for the first fraction is 100 and for the second fraction is 200. We need to calculate the weight average molecular weight of this polythene sample.
Formula to Use
The weight average molecular weight (Mw) of a polymer sample can be calculated using the following formula:
Mw = Σ(wiMi) / Σ(wi)
where,
- wi = weight fraction of the ith polymer molecule
- Mi = molecular weight of the ith polymer molecule
Solution
Let us assume that the total weight of the polythene sample is 1 unit. Then, the weight of the first fraction will be 2/5 units (since the ratio of the two fractions is 2:3) and the weight of the second fraction will be 3/5 units.
Now, we can use the weight average molecular weight formula to calculate the Mw of the polythene sample:
Mw = [(2/5) x 100] + [(3/5) x 200] = 40 + 120 = 160
Therefore, the weight average molecular weight of the polythene sample is 160.
However, this is not the final answer since the molecular weight is usually expressed in thousands. Therefore, we need to multiply this value by 1000:
Mw = 160 x 1000 = 160000
Finally, we need to divide this value by the number of repeat units in the polymer chain, which is 35 for polyethylene:
Mw = 160000 / 35 = 4571.43 ≈ 4600
Therefore, the weight average molecular weight of the polythene sample is approximately 4600.