Q33 / 160 · TS EAMCET 2019 (03 May Shift 1) · Chemistry

A polymer contains 50 molecules with molecular mass 5000, 100 molecules with molecular mass 10,000 and 50 molecules with molecular mass 15,000. Calculate number average molecular mass?

  • A
    5,000
  • B
    75,000
  • Correct answer
    C
    10,000
  • D
    20,000

Solution

$\because$ Number average molecular mass $(M n)$ $$ =\frac{N_2 M_2+N_2 M_2+N_3 M_3}{N_2+N_2+N_3} $$ where, $N_2, N_2$ and $N_3$ are number of molecules and $M_2, M_2$ and $M_3$ are respectively their molecular masses. Given, $$ \begin{aligned} & N_2=50 \text { and } M_2=5000 \\ & N_2=100 \text { and } M_2=10,000 \\ & N_3=50 \text { and } M_3=15,000 \end{aligned} $$ Thus, $$ \begin{aligned} M \bar{n} & =\frac{N_1 M_1+N_2 M_2+N_3 M_3}{N_1+N_2+N_3} \\ & =\frac{(50 \times 5000)+(100 \times 10,000)+(50 \times 15000)}{50+100+50} \\ & =\frac{(25,0000)+(1,000,000)+(75,00,00)}{200} \\ M \bar{n} & =10,000 \end{aligned} $$ Hence, option (3) is the correct answer.

Hence, the correct answer is option: 'C'.

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