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Question

Assertion:
¯Mn value is always greater than ¯Mw value for synthetic polymers.
Reason:
The mass average molecular weight of the polymer is equal to ¯Mw.

A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
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B
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
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C
Assertion is correct but Reason is incorrect
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D
Assertion is incorrect but Reason is correct
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Solution

The correct option is D Assertion is incorrect but Reason is correct
Mn, the number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and Mw, the weight average molecular weight, is calculated from the weight fraction distribution of different sized molecules.

weight-average calculation puts equal statistical weights to each of the equal mass-fractions in the distribution, while a number-average puts equal weights to each of the equal number-fractions, and given two equal number-fractions, the one with the higher molecular mass molecules will be of a greater mass-fraction than the other

Thus, Assertion is incorrect but Reason is correct

Hence, the correct option is D

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