To make the equation demonstrate the Law of Conservation of Mass, we need to ensure that the number of atoms for each element on the reactant side equals the number of atoms for each element on the product side.
The original equation is: \[ 4 \text{Fe} + \text{O}_2 \rightarrow 2 \text{Fe}_2\text{O}_3 \]
On the reactant side:
- Fe: 4 atoms
- O: 2 atoms (from O2)
On the product side:
- Fe: 4 atoms (from 2 Fe2O3)
- O: 6 atoms (from 2 Fe2O3, since each Fe2O3 has 3 O)
Currently, there are 2 oxygen atoms on the reactant side and 6 on the product side, which does not satisfy the conservation of mass.
To balance the equation, we can put a coefficient of 3 in front of O2 on the reactant side. This gives us:
\[ 4 \text{Fe} + 3 \text{O}_2 \rightarrow 2 \text{Fe}_2\text{O}_3 \]
Now, let's count again:
On the reactant side:
- Fe: 4 atoms
- O: 6 atoms (3 O2)
On the product side:
- Fe: 4 atoms (from 2 Fe2O3)
- O: 6 atoms (from 2 Fe2O3)
This new equation now satisfies the Law of Conservation of Mass.
So, the correct response is: On the reactant side, put a 3 in front of O2.