Buna-S contains styrene and buta-1,3-diene units in the mole ratio 1 : 3 (end groups ignored; C 12, H 1). The mass percentage of carbon in the copolymer is

Answer: C

Styrene unit

C X_{8} H X_{8}

and three

C X_{4} H X_{6}

units give

C X_{20} H X_{26}

of mass 266. Carbon = 240/266 x 100 = 90.2 percent. The others are the values for ratio 1:1, pure butadiene and pure styrene.