Step 1: Write the unbalanced chemical equation.
Al + O2 → Al2O3

Step 2: Make a list of the number of atoms of each element on reactant and product side.






Step 3: Pick an element and find out its least common multiple (LCM). Let us pick oxygen. LCM for 2 and 3 is 6.









Step 4: Divide the LCM with the number of atoms of that element and put the result as the coefficient on opposite sides.

Step 4a: There are 2 oxygen atoms on the reactant side; 6 divided by 2 is 3, so put 3 as the coefficient of oxygen on the reactant side.
Al + 3O2 → Al2O3





Step 4b: There are 3 oxygen atoms on the product side; 6 divided by 3 is 2, so put 2 as the coefficient of oxygen on the reactant side.
Al + 3O2 → 2Al2O3





Oxygen is balanced!

Step 5: Repeat Step 3 and 4 for aluminium.
4Al + 3O2 → 2Al2O3





Step 6: Balanced chemical equation.

Both reactant and product sides have equal number of atoms for each element. The chemical equation is now balanced!

Reactants Products
Al = 1 Al = 2
O = 2 O = 3
Reactants Products
Al = 1 Al = 2
O = 2 × 3 = 6 O = 3
Reactants Products
Al = 1 Al = 2 × 2 = 4
O = 2 × 3 = 6 O = 2 × 2 = 6
Reactants Products
Al = 1 × 4 = 4 Al = 4
O = 6 O = 6
LCM of 2 and 3

2 2, 3
3 1, 3
1, 1

2 × 3 = 6
Observation
Question

Tap on the button to fill up the table and then use the + and - buttons to change the coefficients of the elements to balance the chemical reaction.

Table for number of atoms
Elements Reactants Products

\(\text{Fe}_2\text{O}_3~+\) \(\text{C} \rightarrow\) \(\text{Fe}~+\) \(\text{CO}_2\)

2
3
1
1
1
2
Fe
C
O
\(\text{Fe}_2\)
\(\text{Fe}\)
\(\text{O}_3\)
\(\text{O}_2\)
\(\text{C}\)
\(\text{C}\)

Identify the value of x : y, if the given chemical equation is balanced.
\(~~~~~~~~~~~~~~~~~~~~\text{xFe}~+~\text{yCl}_2 \rightarrow \text{2FeCl}_3\)

1 : 2
2 : 1
2 : 3
3 : 2

Solution

Reactants Elements Products
2 Fe 2
6 Cl 6

The balanced chemical equation for the reaction of iron with chlorine is:
\(~~~~~~~~~~~~\text{2Fe}~+~\text{3Cl}_2 \rightarrow \text{2FeCl}_3\)
\(~~~~~~\) Thus, the value of x : y is 2 : 3.

You have successfully learnt that to balance a chemical equation all the elements must have equal atoms on reactant and product sides.