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A reliable method to balance any equation, shown on three worked examples from easy to tricky.
THE RULE: Atoms are never created or destroyed. The number of each type of atom must be the SAME on both sides. You may only change the big numbers in front (coefficients) — never the small subscripts inside a formula.
METHOD
1) Count atoms of each element on both sides.
2) Balance metals first, then non-metals, then hydrogen, then oxygen last.
3) Recount. Adjust. Repeat until every element matches.
EXAMPLE 1: H2 + O2 -> H2O
O: 2 on left, 1 on right -> put 2 before H2O: H2 + O2 -> 2 H2O
Now H: 4 on right, 2 on left -> put 2 before H2: 2 H2 + O2 -> 2 H2O. Balanced.
EXAMPLE 2: Fe + O2 -> Fe2O3
Balance Fe and O by trial: 4 Fe + 3 O2 -> 2 Fe2O3.
Check: Fe = 4 = 4, O = 6 = 6. Balanced.
EXAMPLE 3: C3H8 + O2 -> CO2 + H2O (combustion of propane)
Carbon: 3 -> 3 CO2. Hydrogen: 8 -> 4 H2O. Oxygen on right = (3×2)+(4×1) = 10 -> 5 O2.
Final: C3H8 + 5 O2 -> 3 CO2 + 4 H2O.
EXAM TIP: Leave oxygen for last — it usually appears in more than one product, so balancing it early forces you to redo everything.