Question

If the average molecular weight of a molecule of nylon 6/6 (used to make nylon fibers) is 14,000 g/mole, what is the value of n, i.e., the number of repeating units required to make a molecule of this size?

Answer 1

The general reaction to do this compound is the following:

nHOOC-(CH₂)₄-COOH + n H₂N-(CH₂)₆-NH₂ → [-OC-( CH₂)₄-CO-NH-(CH₂)₆-NH-]_n + 2n H₂O

So if we calculate the inner molecular weight, where it repeats that, and the average MW we can actually calculate the number n so:

[-OC-( CH₂)₄-CO-NH-(CH₂)₆-NH-] in this molecule we have Carbon, Oxygen, nitrogen and hydrogen and each repeat unit has twelve carbons, twenty-two hydrogens, two nitrogens, and two oxygens so:

MW = (12)(12.01 g/mol) + (22)(1.008 g/mol) + (2)(14.01 g/mol) + (2)(16.00 g/mol) = 226.32 g/mol

To get the repeat unit:
n = Avg MW / MW calculated
n = 14000 g/mol / 226.32 g/mol = 61.86

For this result we can round it to 62.

Hope this helps