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JEE Main levelSingle correct optionDifficulty 2 of 3pl136

A sample has equal numbers of chains of molar mass $10000 g mo l^{- 1}$ and $30000 g mo l^{- 1}$ . Using $M_{w} = \sum N_{i} M_{i}^{2} / \sum N_{i} M_{i}$ , its weight-average molar mass in $g mo l^{- 1}$ is

A15000
B20000
C30000
D25000

Answer: D

With

N_{1} = N_{2} = N

:

M_{w} = N (1000 0^{2} + 3000 0^{2}) / [N (10000 + 30000)] = (1 \times 1 0^{8} + 9 \times 1 0^{8}) /40000 = 25000

. The value

20000

is

M_{n}

.

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