how many grams of ethylene (C2H4) are required for the complete reaction of 31.7 grams of hydrogen gas?

Answer 1

C2H4 + H2 ==> C2H6
mols H2 gas = 31.7/molar mass.
1 mol H2 = 1 mol C2H4 from the coefficients in the balanced equation.
g C2H2 = mols C2H4 x molar mass C2H4