Question

How does the law of conservation of mass apply to this reaction: C2H4+O2=H2O+Co2?

Answer 1

Answer:  Only the oxygen needs to be balanced.  There are equal numbers of hydrogen and carbon.

Explanation:

Answer 2

Consider the formation of water molecule. Hydrogen combines with oxygen to form  a water molecule.

 

In this case, the total mass of the reactants = total mass of the products. Also, the number of atoms of hydrogen and oxygen in the reactants side and the products side are equal.

that's all I know