The law of conservation of mass must be followed when balancing chemical equations. This law states that matter cannot be created or destroyed, only transformed. In a chemical reaction, the total mass of the products must equal the total mass of the reactants. This means that the number of atoms of each element must be the same on both sides of the equation.

To balance a chemical equation, coefficients are added to the reactants and products. Coefficients tell us how many molecules or moles of each substance are involved in the reaction. By adjusting the coefficients, we can make sure that the equation obeys the law of conservation of mass.

Here is an example of how to balance a chemical equation:

```

2H2 + O2 -> H2O

```

In this equation, there are two molecules of hydrogen on the left side and one molecule of oxygen on the right side. To balance the equation, we need to add a coefficient of 2 to the water molecule:

```

2H2 + O2 -> 2H2O

```

Now, there are four hydrogen atoms on both sides of the equation and two oxygen atoms on both sides. The equation is balanced.