It is given that total amount of oxygen in the reactants is 4 atoms to balance this chemical equation it is necessary that total number of reactant molecules should equal the total number of products.
Step-by-step explanation:
The coefficients are found by counting each individual atom and atom numbers are same in reactants and products.
Without balancing the equation the formation is H2O and O2. The total number of oxygen in the product side is 3. Hence to balance this equation multiply H2O by 2 and O2 by 1. Thus the total of 4 O2 atoms will be formed on the product side.
Therefore, We can say that the coefficient 2 should be placed in front of water and and 1 in front of oxygen.