Answer
An average of 9,332 of both repeat unit types per molecule.
Work Step by Step
Given:
The number-average molecular weight of a poly(acrylonitrile-butadiene) alternating copolymer is 1,000,000 g/mol
Required:
The average number of acrylonitrile and butadiene repeat units per molecule
Solution:
Each repeat unit of acrylonitrile consists of 3 carbons, 3 hydrogens and 1 nitrogen while butadiene consists of 4 carbons and 6 hydrogens.
The combined repeat unit weight of the two copolymer is:
$m = 7(A_{C}) +9(A_{H}) +1(A_{N})= 7(12.011 g/mol) +9(1.008 g/mol) + 1(14.007 g/mol) = 107.16 g/mol$
From Equation 14.6:
$DP = \frac{M_{n}}{m} = \frac{(1000000 g/mol)}{(107.16 g/mol)} = 9,331.84 \approx 9,332 $
