The law of conservation of mass states that in a closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. This is because atoms are neither created nor destroyed during a chemical reaction, they are simply rearranged.

Here's how you can use this law to balance a chemical equation:

1. Identify the reactants and products: Write down the chemical formulas of all the reactants and products involved in the reaction.

2. Count the atoms of each element: For each element, count the number of atoms present on the reactants' side and the products' side of the equation.

3. Adjust coefficients: Use coefficients (numbers in front of the chemical formulas) to balance the number of atoms of each element on both sides of the equation. Remember, coefficients multiply the entire chemical formula.

4. Check the balance: After adjusting the coefficients, double-check that the number of atoms of each element is the same on both sides.

Example:

Let's balance the equation for the combustion of methane:

CH₄ + O₂ → CO₂ + H₂O

* Reactants: CH₄ (methane) and O₂ (oxygen)

* Products: CO₂ (carbon dioxide) and H₂O (water)

Counting Atoms:

* Reactants:

* C: 1

* H: 4

* O: 2

* Products:

* C: 1

* H: 2

* O: 3

Balancing:

* To balance the hydrogen atoms, put a coefficient of 2 in front of H₂O:

CH₄ + O₂ → CO₂ + 2H₂O

* Now we have 4 H atoms on both sides, but we have 4 oxygen atoms on the product side and 2 on the reactant side. To balance this, put a coefficient of 2 in front of O₂:

CH₄ + 2O₂ → CO₂ + 2H₂O

Now the equation is balanced:

* Reactants:

* C: 1

* H: 4

* O: 4

* Products:

* C: 1

* H: 4

* O: 4

The law of conservation of mass is essential for balancing chemical equations because it ensures that the same number of atoms of each element are present on both sides of the equation, representing the reality of no atom being created or destroyed during a chemical reaction.