Question

In a polymer sample $30\%$ of molecules have a molecular mass of $20,000$, $40\%$ have $30,000$ and the rest $60,000$. What is the weight average molecular mass of the polymer?

• A. $40,300$
• B. $30,600$
• C. $43,333$
• D. $50,400$

Answer 1

Answer: (C)

$43,333$

Weight average Mass $=\frac{\Sigma NiMi}{\Sigma NiMi}$

$\begin{array}{c}\Rightarrow \frac{30\times {\left(20000\right)}^2+40{\left(20000\right)}^2+30\times {\left(60000\right)}^2}{30\times 20000+40\times 30000+30\times 60000}\Rightarrow \left(\frac{30\times 4+40\times 9+30\times 36}{30\times 2+40\times 3+30\times 6}\right)\\ \Rightarrow \frac{120+360+1080}{60+120+180}\times 104\Rightarrow \frac{1560}{360}\times 104\Rightarrow 4.3333\\ Wav\Rightarrow 43333\end{array}$

Option C is correct.