Balancing Chemical Equations

Conservation of Mass and Mole Ratios

Related Tools

Related Labs

Balancing chemical equations is the application of the law of conservation of mass: in any chemical reaction, atoms are neither created nor destroyed. An unbalanced equation may correctly show which substances react and which are produced, but the coefficients (the numbers in front of each formula) must be adjusted so that every element has the same total atom count on both sides. Only coefficients may be changed — subscripts within a formula are fixed by the compound's identity.

The inspection (trial and error) method works by focusing on one element at a time, often starting with elements that appear in only one reactant and one product. The mole ratios implied by the balanced coefficients are central to stoichiometry: a coefficient of 2 in front of H₂O means 2 mol of water, and all other quantities in the reaction scale proportionally. Mastering balancing is the gateway to predicting masses, volumes, and concentrations in quantitative chemistry.

Key Facts

Law of conservation of mass: atoms are neither created nor destroyed in a reaction
Only coefficients (not subscripts) are changed when balancing an equation
Coefficients represent mole ratios of reactants and products
A balanced equation must have equal numbers of each element on both sides
Polyatomic ions (SO₄²⁻, NO₃⁻) can be balanced as a unit if they appear intact on both sides
Combustion reactions: balance C first, then H, then O last (O₂ is easiest to adjust)

Vocabulary

Coefficient: A number placed in front of a chemical formula in an equation that indicates how many moles (or molecules) of that substance are involved.
Subscript: A number written below and to the right of an element symbol in a formula, indicating the number of that atom in one formula unit.
Mole ratio: The ratio of moles of one substance to moles of another substance in a balanced chemical equation; used for stoichiometric calculations.
Reactant: A starting material consumed in a chemical reaction; written on the left side of a chemical equation.
Product: A substance formed during a chemical reaction; written on the right side of a chemical equation.

Common Mistakes to Avoid

Changing subscripts to balance an equation. Changing subscripts changes the identity of the compound (H₂O becomes H₂O₂, which is hydrogen peroxide, not water). Only coefficients may be adjusted.
Forgetting to multiply subscripts by the coefficient. In 2H₂O, there are 4 H atoms and 2 O atoms — the coefficient applies to the entire formula unit.
Balancing elements in the wrong order. Balance elements that appear in only one reactant and one product first. Leave oxygen and hydrogen for last when possible.
Assuming whole-number coefficients are always immediately obvious. Sometimes you need fractions first (e.g. balance ½O₂ then multiply everything by 2 to clear the fraction).

Practice Questions

1 Balance the following equation: Fe + O₂ → Fe₂O₃
2 Balance the combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O. How many moles of O₂ are needed to burn 5 mol of propane?
3 Why is it incorrect to change the subscript in H₂O to H₃O to balance an equation? What compound would H₃O represent instead?