What is the percentage of composition of hydrogen in C3H4?

Ethylene- C2H4

Find the atomic masses for each element and multiply it by the number of atoms in the compound, then add.

C- 12.0 * 2= 24.0

H- 1.00 * 4= 4.00

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28.0

Take the masses for each element and divide it by the total mass. Then change the answer to get the percent.

C 24.0 / 28.0= .857 = 85.7%

H 4.00 / 28.0= .143 = 14.3%

Ethylene is 85.7% Carbon and 14.3% Hydrogen