Verified answer

Дано: 

m(СН2=С(СН3)-СН3) = 7 g

m(p-pa Br2) = 500 g

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w(Br2) - ?

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СН2=С(СН3)-СН3 + Br2 = СН2Br-BrС(СН3)-СН3

M(СН2=С(СН3)-СН3) = 4*12 + 6*1 = 56 g/mol

M(Br2) = 80 * 2 = 160 g/mol

n(СН2=С(СН3)-СН3) = m(СН2=С(СН3)-СН3) / M(СН2=С(СН3)-СН3) =

= 7 / 56 = 1/8 = 0.125 mol

n(СН2=С(СН3)-СН3) = n(Br2) =>

n(Br2) = 0.125 mol

m(Br2) = n(Br2) * M(Br2) = 0.125 * 160 = 20 g

w(Br2) = m(Br2) / m(p-pa) = 20/500 = 0.04 = 4 % mass.

Ответ: 4% масс.