Molar Ratio – IGCSE Chemistry Definition

The molar ratio is the ratio of the amounts (in moles) of reactants and products in a balanced chemical equation. The coefficients (large numbers in front of each formula) give the molar ratio directly. For example, in the equation N₂ + 3H₂ → 2NH₃, the molar ratio of nitrogen to hydrogen to ammonia is 1 : 3 : 2. This means 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia.

Why it matters

The molar ratio is the bridge in every stoichiometric calculation. Once you know the moles of one substance, the molar ratio tells you the moles of any other substance in the reaction. Without it, you cannot convert between reactants and products.

The standard calculation method at IGCSE follows three steps. First, calculate the moles of the known substance from its mass (moles = mass / Mr). Second, use the molar ratio to find the moles of the target substance. Third, convert those moles to mass (mass = moles x Mr), volume, or concentration as required.

Reading the ratio from an equation

The equation must be balanced before you can read the ratio. In 2Fe₂O₃ + 3C → 4Fe + 3CO₂, the ratio of Fe₂O₃ to C to Fe to CO₂ is 2 : 3 : 4 : 3. If you have 0.5 mol of Fe₂O₃, the ratio tells you that you need 0.5 x (3/2) = 0.75 mol of C and will produce 0.5 x (4/2) = 1.0 mol of Fe.

Exam context

Nearly every Paper 4 calculation question requires candidates to state the molar ratio as part of their working. Mark schemes typically award a separate mark for correctly identifying the ratio or using it to convert moles. Skipping this step loses an easy mark.

Worked exam question

In the reaction 2Al + 3CuSO₄ → Al₂(SO₄)₃ + 3Cu, calculate the mass of copper produced when 5.4 g of aluminium reacts completely. (Ar: Al = 27, Cu = 64) (3 marks)

Moles of Al = 5.4 / 27 = 0.20 mol [1]. Molar ratio of Al to Cu = 2 : 3, so moles of Cu = 0.20 x (3/2) = 0.30 mol [1]. Mass of Cu = 0.30 x 64 = 19.2 g [1].

Common mistakes

The most frequent error is using the molar ratio upside down. If the ratio is 2 : 3 and you have moles of the substance with coefficient 2, you must multiply by 3/2, not 2/3. Always put the target substance’s coefficient on top. Another mistake is reading coefficients from an unbalanced equation.