Calculate the number average molecular mass of a polymer having four different monomers A, B, C and D present in equal number. The molecular masses of the monomers are 10000, 15000, 30000 and 50000 respectively.

${\mathrm{M}}_{\mathrm{n}}=\frac{\sum _{ }{\mathrm{M}}_{\mathrm{i}}{\mathrm{N}}_{\mathrm{i}}}{\sum _{ }{\mathrm{N}}_{\mathrm{i}}}$

Here, M_n is the number average molecular mass of a polymer, N_i is the number of molecules of the sample polymer and M_i is the molar mass of the molecule present in the polymer sample.

The polymer made from 4 monomers, A, B, C and D, which are present in equal numbers. Therefore, each monomer has 25% contribution in the polymer.

$M_n=\frac{\left(25\times 10000\right)+\left(25\times 15000\right)+\left(25\times 30000\right)+\left(25\times 50000\right)}{25+25+25+25}=26250$