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JEE Main 2018 · Chemistry · Biomolecules and Polymers · medium

A sample of the copolymer Buna-S, made only from buta-1,3-diene (C4H6) and styrene (C8H8), analyses 90.0% carbon by mass (C=12, H=1). The molar ratio…

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A sample of the copolymer Buna-S, made only from buta-1,3-diene (C4H6\mathrm{C_4H_6}) and styrene (C8H8\mathrm{C_8H_8}), analyses 90.0%90.0\% carbon by mass (C=12\mathrm{C}=12, H=1\mathrm{H}=1). The molar ratio of butadiene to styrene units in the copolymer is
  1. A. 1:41:4
  2. B. 1:11:1
  3. C. 4:14:1
  4. D. 2:12:1
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Answer: (C) 4:14:1

Let the copolymer contain xx mol butadiene (mass 54x54x, C mass 48x48x) and yy mol styrene (mass 104y104y, C mass 96y96y). Then 48x+96y54x+104y=0.900\dfrac{48x+96y}{54x+104y}=0.900, giving 48x+96y=48.6x+93.6y48x+96y=48.6x+93.6y, so 2.4y=0.6x2.4y=0.6x and x:y=4:1x:y=4:1. Check: x=4,y=1x=4,y=1 gives 288/320=0.900288/320=0.900. (Pure butadiene is 88.9%88.9\% C and pure styrene 92.3%92.3\%, so 90%90\% must be nearer to butadiene.)
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