Chemical calculations: moles, reacting masses and concentration
A balanced equation gives a ratio of particles and moles, not a ratio of masses. The reliable route is mass or volume → moles → mole ratio → requested quantity. Keep the units visible at every arrow.
Choose the method
Use n = mass/molar mass. Obtain molar mass by adding the relative atomic masses for every atom in the formula.
Use the coefficients in the balanced equation to convert between reactant and product amounts. Check which reactant is limiting if both amounts are supplied.
For concentration in mol/dm³, use n = cV with V in dm³. Divide a volume in cm³ by 1,000. Use a gas molar volume only at the conditions specified in the question.
Worked examples
Example 1
What mass of MgO forms from 2.4 g of Mg in excess oxygen? Use Mg = 24 and O = 16; assume complete reaction.
Balanced equation: 2Mg + O₂ → 2MgO.
n(Mg) = 2.4/24 = 0.100 mol. The Mg:MgO ratio is 1:1, so n(MgO) = 0.100 mol.
Molar mass of MgO = 40 g/mol. Mass = 0.100 × 40 = 4.0 g.
Example 2
Find the amount of solute in 25.0 cm³ of 0.200 mol/dm³ sodium hydroxide.
Volume = 25.0/1,000 = 0.0250 dm³.
Amount = cV = 0.200 × 0.0250 = 0.00500 mol.
The answer is an amount in mol, not a concentration.
Try it yourself
CaCO₃ → CaO + CO₂. Find the mass of CO₂ released by complete decomposition of 5.00 g CaCO₃. Use molar masses 100 and 44 g/mol respectively.
Show the worked answer
n(CaCO₃) = 5.00/100 = 0.0500 mol.
The ratio is 1:1, so mass of CO₂ = 0.0500 × 44 = 2.20 g.
Common mistakes
Do not multiply grams directly by equation coefficients.
A 1,000-fold error often comes from putting cm³ into n = cV when c is in mol/dm³.
Sources and review
SEAB 2026 syllabus: 2026 Combined Science chemistry component: chemical calculations; also useful for Pure Chemistry revision.
Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.
What the syllabus expects
Give the element symbols and the formulae of the compounds referred to throughout the syllabus.
Determine the formulae of simple compounds from the ratio of atoms present, and the reverse.
Work out ionic compound formulae from the charges carried by the ions, and the reverse.
Read chemical equations that include state symbols.
Build chemical equations, ionic equations included, complete with state symbols.
Define relative atomic mass, Ar.
Define relative molecular mass, Mr, and find it (along with relative formula mass) by adding up the relative atomic masses.
Define the mole by reference to the Avogadro constant.
Given suitable data, compute the percentage by mass of an element within a compound.
Determine empirical and molecular formulae from the data provided.
Compute reacting masses and gas volumes from stoichiometry, taking one mole of any gas as 24 dm3 at room temperature and pressure; questions may involve the idea of limiting reactants. Scope: The gas laws and finding gas volumes at other temperatures and pressures are not required.
Apply the concept of a solution's concentration, given in mol/dm3 or g/dm3, when handling the results of volumetric work such as titrations and when solving simple problems. Scope: Guidance will be given wherever unfamiliar reactions are involved.
Work out percentage yield and percentage purity.
How it's examined
Questions on this topic most often ask you to name, find, calculate, express. About 12% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Rae