Question

If $N_1,N_2,N_3,....$ are the number of molecules with molecular masses $M_1,M_2,M_3,...$ respectively, then mass average molar mass is expressed as :

• A. $\frac{\sum NiM{i}^2}{\sum NiMi}$
• B. $\frac{\sum NiMi}{\sum Ni}$
• C. $\frac{\sum M{i}^2}{\sum Ni}$
• D. $\frac{\sum NiMi}{\sum Mi}$

Answer 1

The correct option is A $\frac{\sum NiM{i}^2}{\sum NiMi}$

Polymer molecules come in different sizes (chain lengths for linear polymers) even ones of the same type.

So, the average molecular mass will depend on the method of averaging. The number average molecular mass is the ordinary arithmetic mean or average of the molecular masses of the individual macromolecules.

Thus, the right equation for mass average molar mass measurement is $\frac{\sum NiM{i}^2}{\sum NiMi}$.