Question

Why is it necessary to balance a chemical equation??

Answer 1

Balancing of chemical equation is necessary because a balanced equation provide us the equal no. of each type of atom on the both side reactant as well as product side.

The Law of Conservation of Mass is the rationale for balancing a chemical equation.

basically balanced equation will provide us the composition of products that how many moles require to complete equation successfully

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Answer 2

 We balance chemical equations because they have to obey law of conservation of mass. 
Law of conservation of mass states that mass can neither created, lost nor destroyed. Therefore we use coefficient in front of reactants and products in order to make sure there are equal number of molecules or atoms on both sides.