A sample of polystyrene is composed of three weight fractions: 0.20, 0.50 and 0.30. The molecular weight of these fractions are 10,000, 40,000 and 60,000, respectively. The weight average molecular weight of this sample is:

Concept:

• The weight average molecular weight is defined as the sum of products of the weight fraction of each type of molecule with the molecular weight of the same type of molecule that is 
• Where, W_i is the weight fraction (fraction of total weight) of each type in polystyrene molecule, M_i is the molecular weight of each type in polystyrene molecule.

Explanation:

Given,

W₁=0.20, W2=0.50, W₃=0.30

M₁=10000, M1=40000, M1=60000

We know, the weight average molecular weight=

 =W₁ M₁ + W2 M2 + W3 M3

 = 0.2× 10000 + 0.5× 40000 + 0.3× 60000

 =40000

Conclusion: -

The weight average molecular weight of the given polystyrene sample is 40000.