Answer:
Explanation: According to law of conservation of mass, mass can neither be created nor be destroyed.
Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
This is an example of combustion reaction in which hydrocarbon reacts with oxygen to form carbon dioxide and water. This is an exothermic reaction in which a large amount of heat is liberated, thus water also exists as vapors.