дано

m(C2H4) = 5 g

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m(C2H6)-?

C2H4+H2-->C2H6

M(C2H4) = 28 g/mol

n(C2H4) = m/M = 5/ 28 = 0.18 mol

n(C2H4) = n(C2H6) = 0.18 mol

M(C2H6) = 30 g/mol

m(C2H6) = n*M = 0.18 * 30 = 5.4 g

ответ 5.4 г