Calculate the masses of substances shown in a balanced symbol equation

The idea

Note that this specific quantitative skill is strictly for Higher Tier (HT) candidates. The method involves three main stages: write the balanced equation; convert the known mass to moles using moles = mass ÷ relative formula mass; use the stoichiometric coefficients to find the moles of the unknown substance; then convert back to mass using mass = moles × relative formula mass. For example, in 2H₂ + O₂ → 2H₂O, 4 g of H₂ is 2 mol, which needs 1 mol of O₂ (32 g) and forms 2 mol of H₂O (36 g). The calculated masses must always be consistent with the law of conservation of mass, meaning the total reactant mass equals the total product mass.

What earns the marks

• Write or use the balanced symbol equation to identify the stoichiometric ratio between the known and unknown substances.
• Calculate the relative formula mass of both the known and unknown substances using the periodic table.
• Convert the given mass of the known substance into moles using the equation: moles = mass ÷ relative formula mass.
• Apply the molar ratio from the balanced equation to determine the moles of the unknown substance.
• Convert the moles of the unknown substance back into mass using mass = moles × relative formula mass, stating the correct units.

Common mistakes and how to put them right

• Forgetting to balance the equation before using the coefficients.

  Put it right by checking that the number of each type of atom is equal on both sides before starting the mole calculation.

• Using the mass ratio directly from the coefficients without converting to moles.

  Put it right by converting to moles first, because equation coefficients represent mole ratios, not mass ratios.

• Including the balancing coefficient when calculating the relative formula mass.

  Put it right by calculating relative formula mass using only the chemical formula (e.g., for 2H₂O, calculate the mass of H₂O as 18, not 36).

• Rounding too early in the calculation.

  Put it right by keeping full values in your calculator until the final step, then rounding to the appropriate number of significant figures.

Exam tips

• Set out your answer as a clear sequence: equation, relative formula masses, moles of known, molar ratio, moles of unknown, mass of unknown.
• Remember that this specific calculation method using moles and balanced equations is a Higher Tier (HT) only requirement.
• If the question gives a mass in tonnes or kilograms, convert it to grams before using relative formula mass, or be prepared to work in standard form.