How to Balance a Chemical Equation

Balancing means making the number of atoms of each element the same on both sides. You change only the coefficients in front of formulas — never the subscripts inside them, because that would change the substance.

The method

1. Write the unbalanced equation and list how many atoms of each element are on each side.
2. Balance one element at a time, starting with an element that appears in only one compound on each side.
3. Leave oxygen and hydrogen for last — they usually appear in several compounds, so they fall into place once everything else is set.
4. Adjust coefficients only. If you need a fraction, multiply the whole equation through to clear it.
5. Check every element, and check that the total charge balances too if ions are involved.

Worked example

Combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O.

• Carbon: 3 on the left, so put 3 in front of CO₂.
• Hydrogen: 8 on the left, so put 4 in front of H₂O (4 × 2 = 8).
• Oxygen last: the right now has 3×2 + 4×1 = 10 O atoms, so put 5 in front of O₂.

Every element now matches, so the equation is balanced.

Common mistakes

• Changing a subscript to balance — never do this; it makes a different compound.
• Forgetting that a coefficient multiplies every atom in the formula.
• Leaving a fraction in the final answer instead of scaling up.

Once the equation is balanced, the coefficients are your mole ratios. Carry the known amount through with the Stoichiometry Calculator, or handle two reactants with the Limiting Reagent Calculator.