Moles (HT only) — AQA GCSE Combined Science
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Moles (HT only) explained
The mole is the chemist's counting unit for particles.
Read the full explanation
Because atoms and molecules are too small to count individually, chemists measure chemical amount in moles. One mole contains the Avogadro constant of particles, 6.02 × 10²³, just as a dozen contains 12. The unit is the mole and its symbol is mol, written after the number (e.g. 2 mol of carbon atoms). Chemical amount is a distinct quantity, measured in moles, not mass or volume. Always give the unit mol in answers. This concept, which is Higher Tier only, lets you compare reacting quantities directly using balanced equations. For example, in 2H₂ + O₂ → 2H₂O, 2 mol of hydrogen molecules reacts with 1 mol of oxygen molecules.
The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
This Higher Tier only statement links the counting unit, the mole, to mass in grams. The relative formula mass, Mr, of a substance is found by adding the relative atomic masses of all atoms in its formula. For water, H₂O, Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. For carbon dioxide, CO₂, Mr = 12 + (2 × 16) = 44, so one mole of carbon dioxide has a mass of 44 g. The numerical value of Mr equals the mass in grams of one mole. This lets you convert between mass and moles using the relationship: amount in moles = mass in grams ÷ molar mass. Always calculate Mr from the formula first, then substitute carefully with units to ensure accurate conversions.
One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance.
A mole is the chemist's counting unit: it is a fixed amount of particles, not a fixed mass. One mole of any substance contains the same number of stated particles as one mole of any other substance, whether those particles are atoms, molecules or ions. For example, 1 mol of helium atoms, 1 mol of H₂ molecules and 1 mol of Na⁺ ions each contain the same number of particles, even though their masses differ. This Higher Tier only concept matters because balanced equations compare particles in fixed ratios, so moles let you convert between reacting masses and particle numbers. The identity of the particle must be stated: 1 mol of oxygen atoms, O, is not the same as 1 mol of oxygen molecules, O₂.
The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 10²³ per mole.
The Avogadro constant is the number of particles in one mole of a substance. Its value is 6.02 × 10²³ per mole, so one mole of any substance contains 6.02 × 10²³ of the stated particles, whether atoms, molecules or ions. To find the number of particles, multiply the number of moles by 6.02 × 10²³. For example, 0.50 mol of water contains 0.50 × 6.02 × 10²³ = 3.01 × 10²³ water molecules. To find the amount in moles from a particle number, divide by 6.02 × 10²³. The particle type must be stated because 1 mol of O₂ contains 6.02 × 10²³ molecules but 1.204 × 10²⁴ oxygen atoms. This is Higher Tier only content, requiring confident use of standard form.
Students should understand that the measurement of amounts in moles can apply to atoms, molecules, ions, electrons, formulae and equations, for example that in one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO₂).
The mole is the unit for amount of substance and applies to any specified particle or formula unit. One mole always contains the Avogadro number of entities, approximately 6.02 × 10²³. The entity must be stated: atoms in C, molecules in CO₂, ions in NaCl, electrons in a charge, or formula units in an equation. For example, 12 g of carbon contains 6.02 × 10²³ C atoms, while 44 g of carbon dioxide contains 6.02 × 10²³ CO₂ molecules, each made of one C atom and two O atoms. Coefficients in a balanced equation give mole ratios, so 2H₂ + O₂ → 2H₂O means two moles of hydrogen molecules react with one mole of oxygen molecules to form two moles of water molecules. This is Higher Tier only content.
Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa.
The relative formula mass, Mr, of a substance is the sum of the relative atomic masses of all atoms in its formula. The mass of one mole of a substance in grams is numerically equal to its Mr. To find moles from mass, use amount in moles = mass in g ÷ Mr. To find mass from moles, rearrange to mass in g = amount in moles × Mr. For example, carbon dioxide has Mr = 12 + (2 × 16) = 44, so 22 g of CO₂ is 22 ÷ 44 = 0.50 mol, and 0.25 mol of CO₂ has mass 0.25 × 44 = 11 g. Always include units and give answers to an appropriate number of significant figures. This relationship applies to any substance when its formula is known. This is Higher Tier only content.
Your focus
- State that chemical amounts are measured in moles and that the unit symbol is mol.
- Describe one mole as containing 6.02 × 10²³ particles.
- Use mole ratios from a balanced equation to compare amounts of reactants and products.
Show all 18 objectives
- Calculate the relative formula mass of a substance from its formula and relative atomic masses.
- State that the mass of one mole in grams is numerically equal to the relative formula mass.
- Convert between mass in grams and amount in moles using molar mass.
- State that one mole of any substance contains the same number of stated particles.
- Identify the particle type, atom, molecule or ion, in a given amount of substance.
- Convert between mass, moles and number of particles using molar mass and the Avogadro constant.
- State the value of the Avogadro constant as 6.02 × 10²³ per mole.
- Calculate the number of particles in a given number of moles.
- Calculate the number of moles from a given number of particles.
- State the number of entities in one mole of a specified substance.
- Identify the entity being counted in a given chemical context.
- Use a balanced equation to determine mole ratios between substances.
- Calculate the relative formula mass of a substance from its formula.
- Convert a given mass of a substance into an amount in moles.
- Convert a given amount in moles of a substance into its mass in grams.
Moles (HT only) exam tips
Marking Points
- States that chemical amount is measured in moles and gives the unit symbol as mol.
- Explains that one mole contains 6.02 × 10²³ particles, so the mole is a counting unit for atoms, molecules or ions.
- Uses the mole to compare reacting quantities from a balanced equation, for example 2 mol H₂ reacting with 1 mol O₂.
- Distinguishes amount in moles from mass in grams and volume in dm³.
- Applies the idea that equal numbers of moles of different substances contain equal numbers of particles.
- Calculates relative formula mass by adding relative atomic masses from the formula, for example Mr of CO₂ = 12 + 2 × 16 = 44.
- States that the mass of one mole in grams is numerically equal to the relative formula mass, so 1 mol CO₂ has mass 44 g.
- Uses molar mass to convert between mass and amount in moles.
- Applies the relationship amount in moles = mass in grams ÷ molar mass to a given substance.
- Handles formulae with brackets or coefficients correctly when finding Mr, for example Ca(OH)₂ = 40 + 2 × (16 + 1) = 74.
- States that a mole is an amount of substance containing a fixed number of stated particles.
- Explains that one mole of any substance contains the same number of particles as one mole of any other substance.
- Identifies particles as atoms, molecules or ions, because the same substance can be described using different particles.
- Recalls that the number of particles in one mole is the Avogadro constant, 6.02 × 10²³ per mole.
- Applies the principle that equal numbers of moles of different substances contain equal numbers of particles but generally have different masses.
- The Avogadro constant is the number of particles in one mole of a substance.
- Its value is 6.02 × 10²³ per mole.
- The particles may be atoms, molecules or ions, and the type must be stated.
- Number of particles = number of moles × 6.02 × 10²³.
- Number of moles = number of particles ÷ 6.02 × 10²³.
- For a molecular substance, the number of atoms is the number of molecules multiplied by the number of atoms in the formula.
- State that one mole of any specified entity contains the Avogadro number of that entity, approximately 6.02 × 10²³.
- Identify the entity being counted: atoms, molecules, ions, electrons, formula units or particles as specified by the question.
- Apply the mole to a formula unit by using the chemical formula to determine the number of each type of atom or ion present in one mole of the substance.
- Use coefficients in a balanced equation to establish mole ratios between reactants and products.
- Explain that equal amounts in moles of different substances contain equal numbers of the specified entities, even though their masses differ.
- Calculate the number of entities by multiplying the amount in moles by the Avogadro constant when required.
- Calculate relative formula mass by summing relative atomic masses from the formula, including any brackets and multipliers.
- State that the mass of one mole of a substance in grams is numerically equal to its relative formula mass.
- Use the equation amount in moles = mass in g ÷ relative formula mass to calculate moles from a given mass.
- Rearrange the equation to mass in g = amount in moles × relative formula mass to calculate mass from a given number of moles.
- Substitute numerical values correctly and evaluate with consistent units, usually grams and moles.
- Round the final answer to an appropriate number of significant figures and include the correct unit, mol or g.
Examiner Tips
- 💡Always attach the unit mol to numerical answers about chemical amount, and never leave a bare number.
- 💡When a question says 'amount of substance', expect moles; when it says 'mass', expect grams, so choose the matching quantity.
- 💡Remember that mole calculations are Higher Tier only, so expect them in more challenging quantitative chemistry questions.
- 💡Show the addition for Mr line by line so the examiner can see how each relative atomic mass was used.
- 💡Write the conversion equation before substituting numbers, then include units in the final answer.
- 💡As this is Higher Tier only, be prepared to use this conversion as part of a larger reacting mass calculation.
- 💡Underline the particle named in the question, such as atoms, molecules or ions, and keep that particle type throughout your working.
- 💡Show the relationship you use, for example moles = mass ÷ molar mass, then substitute values with units.
- 💡Remember this is Higher Tier only content, so expect to apply it in multi-step calculations involving the Avogadro constant.
- 💡Write the Avogadro constant in standard form as 6.02 × 10²³ per mole before substituting values.
- 💡Give answers in standard form to the same number of significant figures as the data, and include the particle unit such as molecules or ions.
- 💡As this is Higher Tier only content, ensure you are confident using standard form on your calculator for these multi-step problems.
- 💡Underline the entity in the question, such as atoms, molecules or ions, and ensure your answer refers to that same entity.
- 💡When using an equation, write the mole ratio beneath the balanced equation before doing any calculation.
- 💡As this topic is Higher Tier only, expect questions to test your understanding of the difference between moles of molecules and moles of atoms within those molecules.
- 💡Write down the formula and calculate Mr as a separate step before substituting into the mole equation.
- 💡Check the rearrangement by substituting simple numbers, for example 44 g ÷ 44 g mol⁻¹ = 1 mol.
- 💡As this is Higher Tier only content, you must be able to rearrange the mole equation confidently to find mass, moles, or relative formula mass.
Common Mistakes
- Writing the unit as 'mole' or 'moles' after a number instead of the symbol mol; correct by writing 3 mol, not 3 moles as a unit label.
- Confusing amount in moles with mass in grams; correct by stating that moles count particles while grams measure mass, and the two are linked by relative formula mass.
- Treating one mole as a fixed mass for every substance; correct by explaining that one mole of carbon has a different mass from one mole of water because the particles differ.
- Forgetting to multiply by the number of atoms in the formula, such as using 16 for CO₂ instead of 32 for the two oxygen atoms; correct by counting every atom in the formula.
- Mixing up the conversion direction, dividing when mass should be multiplied by molar mass; correct by checking units so that grams ÷ g mol⁻¹ gives mol.
- Writing the molar mass without units or with the wrong unit; correct by giving g/mol for molar mass and g for mass.
- Treating one mole as a fixed mass such as 1 g or 12 g; correction: a mole is a fixed number of particles, and its mass depends on the substance.
- Assuming 1 mol of oxygen atoms and 1 mol of oxygen molecules contain the same number of oxygen atoms; correction: 1 mol of O₂ contains 2 mol of oxygen atoms.
- Comparing moles of different substances by mass alone; correction: convert mass to moles using molar mass before comparing particle numbers.
- Writing the Avogadro constant as 6.02 × 10²² or 6.02 × 10²⁴; correction: the value is 6.02 × 10²³ per mole.
- Multiplying by the Avogadro constant when converting particles to moles; correction: divide the particle number by 6.02 × 10²³.
- Forgetting to multiply by the number of atoms in a molecule; correction: 1 mol of O₂ contains 2 × 6.02 × 10²³ oxygen atoms.
- Treating one mole of a compound as one mole of each element within it; correct by using the formula to find the number of atoms or ions per formula unit, for example one mole of CO₂ contains two moles of oxygen atoms.
- Confusing the mass of one mole with the number of entities; correct by separating relative formula mass in grams from the Avogadro number of entities.
- Omitting the entity when stating a mole amount; correct by always naming what is counted, such as moles of electrons or moles of water molecules.
- Using relative atomic mass instead of relative formula mass for a compound; correct by summing all atoms in the formula, for example Mr of H₂O = 2 × 1 + 16 = 18.
- Dividing Mr by mass instead of mass by Mr; correct by checking that the rearrangement gives moles as the subject.
- Forgetting to multiply by a subscript or bracket multiplier; correct by expanding the formula carefully, such as Mr of Ca(OH)₂ = 40 + 2 × (16 + 1) = 74.