A-Level chemistry calculations: start with quantities and units

Three relationships to organise your thinking

Amount in moles = mass ÷ molar mass. For a solution, amount in moles = concentration × volume, when concentration is in mol dm⁻³ and volume is in dm³. A balanced reaction then tells you the ratio of reacting amounts in moles.

These ideas sit within amount-of-substance work in AQA’s A-Level chemistry specification. Check your own awarding body’s specification for the content and assessment requirements that apply to you.

Example 1: mass to amount in moles

Question: What amount is present in 5.85 g of sodium chloride? Use a molar mass of 58.5 g mol⁻¹.

The units help explain the operation: dividing grams by grams per mole leaves moles. As a check, one tenth of the molar mass should give one tenth of a mole.

Example 2: solution concentration and volume

Question: How many moles are in 25.0 cm³ of a 0.200 mol dm⁻³ solution?

Using 25.0 without converting the volume would make the answer 1000 times too large. Write the conversion as part of the working so it is easy to check.

Example 3: use the balanced equation

For the reaction Mg + 2HCl → MgCl₂ + H₂, one mole of magnesium reacts with two moles of hydrogen chloride.

If 0.0200 mol of magnesium reacts completely with sufficient acid, it requires 0.0400 mol of HCl and produces 0.0200 mol of hydrogen. The coefficients give mole ratios, not direct mass ratios.

If a problem supplies amounts for both reactants, check which limits the reaction before calculating the product. Do not assume the other reactant is in excess unless the question supports it.

Try a fresh question

Calculate the amount in 40.0 cm³ of a 0.150 mol dm⁻³ solution. Write the unit conversion before multiplying.

These are original worked examples, not questions taken from an exam paper. Numerical answers are given to three significant figures to match the example data.

Find the step that needs support

If the calculation is still difficult, identify whether the uncertainty is about the chemistry, selecting a relationship, rearranging it, converting units or using a ratio. That is a clearer starting point than trying more long questions without feedback.

Explore one-to-one A-Level chemistry tuition to discuss support with concepts and calculations. We confirm your course and a suitable tutor before agreeing lessons.