Home / Subjects / Chemistry
⚗️ Chemistry

Moles and Molar Mass: Calculations Explained Step by Step

Convert between mass and moles using Mr, use balanced equations to find reacting masses and calculate concentration, with worked examples.

  • ⏱ 3 min read
  • ✏️ 3 worked examples
  • 🎯 5 practice questions
  • 📶 Intermediate
GCSEA-LevelHigh SchoolAP
Start practice questionsDownload PDF

Curriculum fit: GCSE Chemistry and Combined Science (quantitative chemistry; mole calculations are mainly Higher tier). US: high school and AP Chemistry (stoichiometry).

On this page

What Is It?

A mole is an amount of substance containing 6.02 × 10²³ particles (Avogadro's number). The mass of one mole in grams equals the relative formula mass (Mr).

Moles let you link the masses of reactants and products in a balanced equation, and work out concentrations of solutions.

massnMrn = moles, Mr = relativeformula massmoles = mass ÷ Mrmass = moles × Mr36 g ÷ 18 = 2 mol
Cover the quantity you want. For example, 36 g of water (Mr 18) is 36 ÷ 18 = 2 moles.

Key Formulas and Ideas

  • moles = mass ÷ Mr (mass in grams).
  • mass = moles × Mr.
  • concentration (mol/dm³) = moles ÷ volume in dm³.
  • Mr = sum of the relative atomic masses of all atoms in the formula.

Worked Examples

Example 1

Mr and Mass

Question

Find the mass of 0.25 mol of calcium carbonate, CaCO₃ (Ar: Ca 40, C 12, O 16).

  1. Mr = 40 + 12 + 3 × 16 = 100.
  2. mass = 0.25 × 100 = 25.

✓ Answer 25 g

Example 2

Reacting Masses

Question

6 g of magnesium burns in oxygen: 2Mg + O₂ → 2MgO. Find the mass of magnesium oxide made (Ar: Mg 24, O 16).

  1. Moles of Mg = 6 ÷ 24 = 0.25.
  2. The ratio Mg : MgO is 1 : 1, so 0.25 mol MgO.
  3. Mr of MgO = 40, so mass = 0.25 × 40 = 10 g.

✓ Answer 10 g

Example 3

Concentration

Question

2 mol of a substance is dissolved to make 4 dm³ of solution. Find the concentration.

  1. Concentration = moles ÷ volume.
  2. 2 ÷ 4 = 0.5.

✓ Answer 0.5 mol/dm³

Common Mistakes to Avoid

  • Using the mass in kilograms instead of grams.
  • Forgetting to use the balanced equation ratio.
  • Not converting cm³ to dm³ by dividing by 1000.
  • Adding Mr values without multiplying by the number of atoms.

Practice Questions

0 of 5 answered
Q1How many moles are in 11 g of CO₂ (Mr = 44)?
Q2What is the mass of 3 mol of water (Mr = 18)?
Q3What is the Mr of NaOH (Na 23, O 16, H 1)?
Q42 mol dissolved in 4 dm³ gives a concentration of…
Q5How many particles are in one mole?

Frequently Asked Questions

Where do I find atomic masses?

On the periodic table. Your exam usually provides one, so check what is given.

What is the formula for moles?

Moles = mass ÷ molar mass, where mass is in grams and molar mass is in g/mol.

Take This Guide With You

Get a free printable PDF with the worked examples and quiz answers. Enter your email and the download starts straight away.

Sources and Review

Reviewed by StudyMorrow editorial review on 5 Nov 2026.