How to Balance a Chemical Equation

The rule

Atoms are not created or destroyed in a reaction, so each element must appear the same number of times on both sides. You balance an equation by changing the coefficients in front of each formula, never the small numbers inside a formula, which would make it a different substance.

Worked example: burning propane

Start with C₃H₈ + O₂ → CO₂ + H₂O.

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

The result is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Balance elements that appear in only one compound on each side first, and leave elements that appear on their own, like O₂, until the end.

Tips

• If you end up with a fraction, multiply every coefficient by the same number to make them whole.
• Treat a polyatomic ion such as SO₄ as one unit if it appears unchanged on both sides.
• In ionic equations the total charge must also be the same on both sides.

Check it instantly

The Chemical Equation Balancer balances any equation, with charges, using the smallest whole numbers. The coefficients give the mole ratios for working out how much product to expect, and the Percent Yield Calculator compares that with what you actually got.

Two more worked examples

Aluminum in hydrochloric acid: Al + HCl → AlCl₃ + H₂.

• Chlorine comes in threes on the right and ones on the left, so look for a multiple of both: 6 HCl gives 6 Cl, enough for 2 AlCl₃.
• That needs 2 Al on the left, and the 6 H become 3 H₂.
• Result: 2Al + 6HCl → 2AlCl₃ + 3H₂.

Rusting iron: Fe + O₂ → Fe₂O₃. Two Fe₂O₃ give 6 oxygen atoms, which is 3 O₂, and 4 Fe: 4Fe + 3O₂ → 2Fe₂O₃.

From coefficients to grams

The coefficients are mole ratios, not mass ratios. Burning 44 g of propane, which is 1 mole, needs 5 moles of O₂: 5 × 32 = 160 g of oxygen, and produces 3 × 44 = 132 g of CO₂. Mass is conserved overall: 44 + 160 = 132 + 72 g of water.

Common mistakes

• Changing a subscript. Turning H₂O into H₂O₂ balances the oxygen and changes water into hydrogen peroxide. Only the coefficients may change.
• Stopping at a fraction. C₂H₆ + 3.5O₂ is balanced but not conventional. Double everything: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
• Balancing oxygen first. It usually appears in several compounds. Leave the element that appears in the most places until last.
• Forgetting the water of a hydrate. CuSO₄·5H₂O carries 5 extra waters, which count on that side of the equation.