Question

If N₁, N₂, N₃, .... are the number of molecules with molecular masses M₁, M₂, M₃, .... respectively, the weight average molecular mass is given by ______.

Options

• `(sum N_i M_i^2)/(sum N_i M_i)`

• `(sum N_i M_i)/(sum N_i)`

• `(sum N_i M_i^2)/(sum M_i)`

• `(sum N_i M_i)/(sum M_i)`

Answer 1

If N₁, N₂, N₃, .... are the number of molecules with molecular masses M₁, M₂, M₃, .... respectively, the weight average molecular mass is given by `bbunderline((sum N_i M_i^2)/(sum N_i M_i))`.

Explanation:

The weight average molecular mass gives more importance to heavier molecules in a polymer sample. It is defined as the ratio of the sum of products of the number of molecules and the square of the molecular mass to the sum of products of the number of molecules and the molecular mass.
Therefore, the correct expression is:

`bar M_w = (sum N_i M_i^2)/(sum N_i M_i)`