Discussion
Determine the mole fraction of vinyl chloride in a copolymer of vinyl chloride (CH2CHCl) and vinyl acetate (CH2-CH-OCOCH3) having molecular weight of 10520 g/mol and degree of polymerization of 160.
Answer: D
The mole fraction of vinyl chloride is about 0.86.
The average mass per repeat unit is 10520/160 = 65.75 g/mol. The repeat unit masses are about 62.5 g/mol for vinyl chloride (C2H3Cl) and 86.1 g/mol for vinyl acetate (C4H6O2). With f the mole fraction of vinyl chloride:
\[ \begin{aligned} 62.5f + 86.1(1 - f) &= 65.75 \\ 23.6f &= 20.35 \\ f &\approx 0.86 \end{aligned} \]
0.14 is the mole fraction of vinyl acetate, the complement. 0.30 and 0.70 do not satisfy the average mass; a 70 percent vinyl chloride copolymer would have a mean repeat mass of about 69.6 g/mol.
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