Ethane (C2H6) on complete combustion gave CO2 and water. It shows that the results are in accordance with the law of conservation of mass. Then, the coefficient of oxygen is equal to
According to law of conservation of mass, the mass of the elements on the product side should be equal to the mass of the elements on reactant side. In simple words, the number of atoms of each element should be same on the both sides. This can be achieved by balancing a chemical reaction.
Unbalanced chemical equation for combustion of ethane:
The balanced chemical equation:
From the balanced chemical equation, we can observe that the coefficient of O2 is 
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2