Question

Which equation obeys the law of conservation of mass?

Answer 1

4Fe + 3O2 -> 2 (Fe2O3)

Both sides have 4Fe and 6O.

Answer 2

Answer: $4Fe+3O_2\rightarrow 2Fe_2O_3$

Explanation:

According to the 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.

Synthesis reaction is a chemical reaction in which two reactants are combining to form one product.

The chemical equation for synthesis of $Fe_2O_3$ from $Fe$ and $O_2$ is:

$4Fe+3O_2\rightarrow 2Fe_2O_3$