How does the law of conservation of mass apply to this reaction: c2h4 + o2 → h2o + co2? a. the equation needs to be balanced. there are fewer oxygen atoms in the equation than hydrogen or carbon. b. only the oxygen needs to be balanced. there are equal numbers of hydrogen and carbon. c. the law of conservation of mass has already been applied. there is an equal number of each element on both sides of the equation. d. each element needs to be balanced.

The correct answer is D. The law of conservation of mass states that in a system matter can neither be created or destroyed, hence atoms of each element on reactant side should equal atoms of each element on product side. In the unbalanced equation:

, the are 2 carbon atoms on reactant side as opposed to 1 carbon atom on product side,  there are 4 hydrogen atoms on reactant side as opposed to 2 hydrogen atoms on product side, and there are 2 oxygen atoms on reactant side as opposed to 3 atoms of oxygen on product side. To balance the equation we add a coefficient of 3 on O_2 and on the product side we put a coefficient of 2 on both water and carbon dioxide .

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