Latimer Tuition

Navigation

Latimer Tuition

GCSE Chemistry guide

GCSE Chemistry moles: mass, Mr and equation ratios

Learn what a mole and Mr mean, use n = m ÷ M with the correct units, and follow the mass → moles → ratio → moles → mass method through worked GCSE examples.

Explore GCSE Chemistry tuitionMatch me with a tutor

Editorial

  • Portrait of Robin GibbonsLead authorFounder & CEO·Latimer Tuition
  • Reviewed byHeather LatimerTutor Matching & Customer Service·Latimer Tuition
  • Published
  • Last reviewed

Current answer

What are moles in GCSE Chemistry?

A mole is the unit used for amount of substance: it counts a fixed number of specified particles rather than measuring mass. Relative formula mass, Mr, is the total of the relative atomic masses shown by a formula and has no unit. Molar mass, M, is the mass of one mole and is usually written in g mol⁻¹.

For one substance, use n = m ÷ M to find moles and m = n × M to find mass. For a reaction, balance the equation first, then follow mass → moles → coefficient ratio → target moles → target mass.

Key terms, symbols and units

Keep the quantity and its unit together. The most important distinction is that Mr is unitless, while molar mass has units.

Meanings and units for the quantities used in GCSE mole calculations.

TermPlain-English meaningUnit

Relative atomic mass, Ar

A relative mass value for an element, normally taken from the periodic table supplied with the question.

None

Relative formula mass, Mr

The sum of the Ar values for every atom represented by a chemical formula.

None

Molar mass, M

Mass per mole of the specified substance.

Usually g mol⁻¹ at GCSE

Amount of substance, n

The quantity measured by counting specified entities in moles.

mol

Mass, m

The mass of the sample used in the calculation.

Usually g when M is in g mol⁻¹

Coefficient

A number before a whole formula. In a balanced equation, coefficients give the relative mole ratio.

None

How to calculate relative formula mass, Mr

According to AQA: “The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.” Use the Ar values supplied in the question or official data because rounding can differ between periodic tables.

  • 1. Write down each element and its Ar

    For Na₂SO₄, use Ar(Na) = 23, Ar(S) = 32 and Ar(O) = 16.

  • 2. Count every atom in the formula

    The subscripts show two sodium atoms and four oxygen atoms. A missing subscript means one atom. If brackets are present, multiply every atom inside the brackets by the number outside.

  • 3. Treat a bracketed group as a whole

    In Ca(OH)₂, the 2 outside the brackets applies to both oxygen and hydrogen. Count one calcium atom, two oxygen atoms and two hydrogen atoms before adding the Ar values.

  • 4. Multiply, then add

    Mr(Na₂SO₄) = (2 × 23) + 32 + (4 × 16) = 46 + 32 + 64 = 142.

  • 5. Give Mr without a unit

    Write Mr = 142. The molar mass is numerically 142 g mol⁻¹ when the same rounded Ar values are used, but Mr itself is a relative value and has no unit.

The mass–moles relationship

“Mass divided by amount of substance.” — IUPAC, defining molar mass

That definition gives the two forms used most often in GCSE Chemistry:

Find moles

n = m ÷ M. With mass in grams and molar mass in g mol⁻¹, the units cancel as g ÷ g mol⁻¹ = mol.

Find mass

m = n × M. The units combine as mol × g mol⁻¹ = g.

Use consistent mass units

When M is in g mol⁻¹, convert the starting mass to grams: 1000 mg = 1 g and 1000 g = 1 kg.

Round at the end

Keep extra calculator digits during intermediate steps. Follow any explicit instruction, then give the final answer to a sensible number of significant figures for the data supplied. There is no single fixed rule for every mole question.

Worked examples: mass to moles and moles to mass

Use the numerical value of the substance’s Mr as its molar mass in g mol⁻¹, while keeping the distinction between the two quantities clear.

Example 1: calculate moles from mass

Question: How many moles are in 20 g of calcium carbonate, CaCO₃? Use Ar(Ca) = 40, Ar(C) = 12 and Ar(O) = 16.

  1. Mr(CaCO₃) = 40 + 12 + (3 × 16) = 100
  2. M(CaCO₃) = 100 g mol⁻¹
  3. n = m ÷ M = 20 g ÷ 100 g mol⁻¹ = 0.20 mol

Answer: 0.20 mol of CaCO₃.

Example 2: calculate mass from moles

Question: What mass is 0.25 mol of carbon dioxide, CO₂? Use Ar(C) = 12 and Ar(O) = 16.

  1. Mr(CO₂) = 12 + (2 × 16) = 44
  2. M(CO₂) = 44 g mol⁻¹
  3. m = n × M = 0.25 mol × 44 g mol⁻¹ = 11 g

Answer: 11 g of CO₂. A quick check is that 0.25 mol is one quarter of a mole, so the mass should be one quarter of 44 g.

Subscripts and coefficients are not the same

Read these numbers correctly before using an equation ratio. Changing a subscript changes the substance; changing a coefficient changes how many particles or moles take part.

Subscript: H₂O

The small 2 is part of the formula. One water molecule contains two hydrogen atoms and one oxygen atom. Keep this formula fixed when balancing an equation.

Coefficient: 2H₂O

The 2 in front multiplies the whole formula. It represents two water molecules or two moles of water. In a balanced equation, coefficients provide the mole ratio.

How to use mole ratios in a balanced equation

When a question moves from one substance to another, use mass → moles → coefficient ratio → target moles → target mass. Do not compare the starting and target masses directly.

  • 1. Identify what is known and what is required

    Mark the starting substance and the target substance, then note whether each quantity is a mass, an amount in moles or something else.

  • 2. Calculate the relevant Mr or molar masses

    Use every atom shown by each formula, including brackets and subscripts.

  • 3. Balance the chemical equation

    Change coefficients only. The coefficients in the balanced equation give the relative amounts in moles.

  • 4. Convert the known mass to moles

    Use n = m ÷ M, with mass in grams when M is in g mol⁻¹.

  • 5. Apply the coefficient ratio

    Scale the known moles from the starting coefficient to the target coefficient. Write this step even when the ratio simplifies to 1:1.

  • 6. Convert the target moles to mass

    Use m = n × M for the target substance.

  • 7. Add the unit and check the answer

    Keep extra digits until the final step, then round as instructed or sensibly. Check that the answer’s size fits the mole ratio and molar masses.

Worked mole-ratio and reacting-mass examples

Both examples show the coefficient step explicitly. The word theoretical means the amount predicted from the balanced equation under the stated assumptions.

Example 1: a 1:1 mole ratio

Question: What theoretical mass of magnesium oxide forms from 4.8 g of magnesium when oxygen is in excess?

Balanced equation: 2Mg + O₂ → 2MgO

Use M(Mg) = 24 g mol⁻¹ and M(MgO) = 40 g mol⁻¹.

  1. Moles of Mg: 4.8 g ÷ 24 g mol⁻¹ = 0.20 mol
  2. Ratio Mg:MgO = 2:2 = 1:1, so moles of MgO = 0.20 mol
  3. Mass of MgO: 0.20 mol × 40 g mol⁻¹ = 8.0 g

Answer: 8.0 g of MgO.

Example 2: a 1:2 mole ratio

Question: What theoretical mass of ammonia forms from 14 g of nitrogen gas when hydrogen is in excess?

Balanced equation: N₂ + 3H₂ → 2NH₃

Use M(N₂) = 28 g mol⁻¹ and M(NH₃) = 17 g mol⁻¹.

  1. Moles of N₂: 14 g ÷ 28 g mol⁻¹ = 0.50 mol
  2. Ratio N₂:NH₃ = 1:2, so moles of NH₃ = 0.50 × 2 = 1.0 mol
  3. Mass of NH₃: 1.0 mol × 17 g mol⁻¹ = 17 g

Answer: 17 g of NH₃. The 1:2 comes from the coefficients, not from subscripts or masses.

Common mole-calculation mistakes

Use the final column as a repair step when your answer does not look right.

Frequent errors in Mr, mass–moles and reacting-mass calculations, with corrections.

Common errorWhy it failsWhat to do instead

Adding each element only once when finding Mr

It ignores the number of atoms represented by subscripts or bracket multipliers.

Count every atom first, then multiply each Ar before adding.

Changing a subscript to balance an equation

A subscript is part of the chemical formula, so changing it creates a different substance.

Keep formulae fixed and alter coefficients only.

Taking a mole ratio before balancing

The coefficients do not yet show the correct conserved proportions.

Balance the equation before reading the coefficient ratio.

Moving directly from one mass to another using the coefficients

Coefficients compare amounts in moles, not usually masses.

Convert the known mass to moles, use the ratio, then convert target moles to mass.

Writing Mr in g mol⁻¹

Mr is a relative, dimensionless value.

Write Mr with no unit; use g mol⁻¹ for molar mass, M.

Multiplying mass by molar mass to find moles

The units do not reduce to moles.

Use the units: g ÷ g mol⁻¹ = mol, so divide to find moles.

Rounding every intermediate answer

Repeated rounding can shift the final result.

Keep extra calculator digits and round the final answer only.

Assuming the reactant with the smallest mass runs out first

Reactants have different molar masses and equation coefficients.

For a limiting-reactant question, compare each amount in moles divided by its coefficient. Treat this as an extension where your course includes it.

Sources and further reading

Scientific definitions come from BIPM and IUPAC. Assessment and qualification points use current official awarding-organisation and regulator information accessed on 4 August 2026.

  • BIPM — SI base unit: mole

    Exact definition of the mole, amount of substance and specified entities. Accessed 4 August 2026.

    Open source 1

  • IUPAC — Gold Book: molar mass

    Definition and units of molar mass. Online version 5.0.0, 2025; accessed 4 August 2026.

    Open source 2

  • AQA — GCSE Chemistry 8462: Quantitative chemistry

    Mr, moles, balanced equations, reacting masses and AQA tier context. Accessed 4 August 2026.

    Open source 3

  • Pearson Edexcel — GCSE Chemistry specification

    Calculation scope, units, multistep working and Edexcel tier context. Issue 4, March 2024; accessed 4 August 2026.

    Open source 4

  • OCR — GCSE Chemistry A J248 specification

    Moles, stoichiometry, mathematical skills, tier context and common misconceptions. Version 3.6, 2024; accessed 4 August 2026.

    Open source 5

  • OCR — data and equation sheets

    Current information about OCR Chemistry data-sheet provision. Published 11 March 2026; accessed 4 August 2026.

    Open source 6

  • Ofqual — current support-sheet decision

    Current national policy for mathematics, physics and combined science. Published 5 May 2026; accessed 4 August 2026.

    Open source 7

  • WJEC — GCSE Chemistry

    Current qualification information and the September 2026 Unit 3 update. Accessed 4 August 2026.

    Open source 8

  • WJEC — separate science GCSE availability

    Current timetable for separate Biology, Chemistry and Physics GCSEs in Wales. Accessed 4 August 2026.

    Open source 9

  • Qualifications Scotland — National 5 Chemistry

    Current National 5 course context for Scotland. Updated 4 June 2026; accessed 4 August 2026.

    Open source 10

  • Latimer Tuition — GCSE Chemistry tuition

    Current subject-support scope. Accessed 4 August 2026.

    Open source 11

  • Latimer Tuition — tutor matching

    Current matching-form process and booking boundary. Accessed 4 August 2026.

    Open source 12

Related guidance

More guidance from this section

More guidance from this part of the Ed Centre that may help with the same decision, stage or next step.

Support and clarity

Frequently asked questions

Straight answers to the questions people ask most often.

What is a mole in chemistry?

A mole is the SI unit of amount of substance, not a mass. One mole contains exactly 6.022 140 76 × 10²³ specified entities, while GCSE work commonly uses the rounded value 6.02 × 10²³. Name the entity where relevant, such as atoms, molecules, ions or formula units.

What is Mr in GCSE Chemistry, and does it have a unit?

Mr is relative formula mass. Add the Ar values for every atom represented in the formula, including the effect of brackets and subscripts. Mr is a relative value and has no unit. Use the Ar values provided with the question because rounding can differ.

Is molar mass the same as Mr?

No. Mr is a unitless relative value; molar mass, M, is mass per mole and is normally written in g mol⁻¹. Their numerical values commonly match when the molar mass is expressed in g mol⁻¹ and the same atomic-mass data are used.

How do you calculate moles from mass?

Convert the mass to grams when the molar mass is in g mol⁻¹, then use n = m ÷ M. In the usual GCSE form, divide the mass in grams by the numerical value of Mr. Show that g ÷ g mol⁻¹ gives mol, keep extra digits during working and round at the end.

How do you convert moles to grams?

Use m = n × M. Multiply the amount in moles by the molar mass in g mol⁻¹; mol × g mol⁻¹ gives a mass in grams.

Why must an equation be balanced before using a mole ratio?

The coefficients in a balanced equation express the correct proportional amounts of the substances. An unbalanced equation does not give the correct stoichiometric ratio. Balance by changing coefficients, never subscripts.

Are mole calculations Foundation or Higher Tier?

It depends on the awarding organisation and exact subtopic. AQA GCSE Chemistry 8462 labels its amount-of-substance sections Higher Tier, while Pearson Edexcel and OCR use their own arrangements. Do not apply one board’s tiering to every GCSE, Combined Science or IGCSE course.

Will the moles equation be supplied in the exam?

Do not assume there is a universal standalone GCSE Chemistry equation sheet. Ofqual’s national support arrangements cover mathematics, physics and combined science rather than a general Chemistry sheet, although individual boards may provide data or information in questions. Use the current materials for your exact awarding organisation, qualification and exam series.

Sources and references

Sources and references

Official guidance

Internal pages