Question

Assertion:

$\overline{M_n}$ value is always greater than $\overline{M_w}$ value for synthetic polymers.

Reason:

The mass average molecular weight of the polymer is equal to $\overline{M_w}$.

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

Answer 1

Answer: (D)

$M_n$, the number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and $M_w$, 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 $\text{D}$