Moles (HT only) — AQA GCSE Chemistry
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Moles (HT only) explained
Moles (HT only) covers 6 specification points, including Chemical amounts are measured in moles.
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The… and The mass of one mole of a substance in grams…. Open a point below for its explanation and revision guidance.
Chemical amounts are measured in moles. The symbol for the unit mole is mol.
The mole is the unit used to measure chemical amount. Its symbol is mol. One mole contains the Avogadro number of particles, 6.02 × 10²³, where the particles may be atoms, molecules, ions or electrons. The mass of one mole of a substance is its molar mass, found by adding the relative atomic masses in the formula and expressed in g/mol. For example, the molar mass of water, H₂O, is 18 g/mol, so 36 g of water is 2 mol. The relationship amount in moles equals mass divided by molar mass allows you to convert between mass and chemical amount. Always write mol after the number, for example 2 mol, not 2 m.
The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
The mole is the chemist's counting unit for particles. For any substance, the relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in its formula. If you take that many grams of the substance, you have exactly one mole of it. For example, water H₂O has Mr = (2 × 1) + 16 = 18, so one mole of water has a mass of 18 g. Carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so one mole of CO₂ has a mass of 44 g. This numerical bridge lets you convert between mass in grams and amount in moles using n = m ÷ Mr, where n is amount in moles, m is mass in grams and Mr is relative formula mass. The same relationship applies to elements, compounds and ions, provided you use the correct formula.
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 a fixed amount of particles, just as a dozen is a fixed count. One mole of any substance contains the same number of stated particles, whether those particles are atoms, molecules, ions or electrons. For example, one mole of helium contains the same number of atoms as one mole of carbon, and one mole of carbon dioxide contains the same number of molecules as one mole of water. The type of particle must be stated because the same amount in moles can refer to different entities. This idea allows chemists to compare amounts fairly in equations and calculations. It also explains why reacting ratios in equations correspond to simple whole-number ratios of moles.
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 10 per mole.
The Avogadro constant is the number of particles in one mole of any substance. Its value is 6.02 × 10²³ per mole. This means that one mole of carbon contains 6.02 × 10²³ carbon atoms, one mole of water contains 6.02 × 10²³ water molecules, and one mole of sodium chloride contains 6.02 × 10²³ formula units of NaCl. The constant links the microscopic world of particles to the macroscopic world of masses we can measure. To find the number of particles in a given amount in moles, multiply the number of moles by 6.02 × 10²³. To find the amount in moles from a number of particles, divide by 6.02 × 10²³. The type of particle must always be stated.
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 (CO ).
The mole is a general unit for counting any specified particle. It can be used for atoms, molecules, ions, electrons, formula units and the substances in equations. For example, one mole of carbon contains 6.02 × 10²³ carbon atoms, and one mole of carbon dioxide contains 6.02 × 10²³ carbon dioxide molecules. The number of particles is the same, but the particles are different. In equations, coefficients show the ratio of moles of each substance. For example, in 2H₂ + O₂ → 2H₂O, two moles of hydrogen molecules react with one mole of oxygen molecules to form two moles of water molecules. The mole can also count electrons, such as in electrolysis or redox reactions, where one mole of electrons is 6.02 × 10²³ electrons.
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 mole is the amount unit linking mass to particles. One mole of a substance has a mass in grams equal to its relative formula mass, Mr. For a pure substance, amount in moles n = mass m ÷ Mr, and mass m = n × Mr. For example, carbon dioxide CO₂ has Mr = 12 + (2 × 16) = 44, so 88 g contains 88 ÷ 44 = 2 mol, and 0.25 mol has mass 0.25 × 44 = 11 g. For a monatomic element, relative atomic mass Ar is used; for a compound or molecular element (like O₂), add Ar values for every atom in the formula. Rearranging the same relationship gives both directions. Always include units: g for mass and mol for amount. Note that Mr is dimensionless, whereas molar mass has units of g mol⁻¹.
Your focus
- State the unit and symbol for chemical amount.
- Describe the number of particles in one mole.
- Calculate the amount in moles from mass and molar mass.
Show all 18 objectives
- Calculate the relative formula mass of a substance from its formula and relative atomic masses.
- Convert between mass in grams and amount in moles using n = m ÷ Mr.
- Explain why the mass of one mole in grams is numerically equal to the relative formula mass.
- State that one mole of any substance contains the same number of stated particles.
- Identify the correct particle type when describing a mole of a given substance.
- Use mole ratios from balanced equations to compare amounts of substances.
- State the value of the Avogadro constant as 6.02 × 10²³ per mole.
- Calculate the number of particles in a given amount in moles.
- Calculate the amount in moles from a given number of particles.
- Describe how the mole can be applied to atoms, molecules, ions, electrons and formulae.
- Use a balanced equation to state the mole ratio of reactants and products.
- Compare the number of stated particles in equal amounts in moles of different substances.
- Calculate the number of moles in a given mass using n = m ÷ Mr.
- Calculate the mass of a given number of moles using m = n × Mr.
- Determine Mr of a compound accurately from its formula and Ar values.
Moles (HT only) exam tips
Marking Points
- State that the mole is the unit of chemical amount and its symbol is mol.
- State that one mole contains 6.02 × 10²³ particles.
- Calculate molar mass by adding relative atomic masses from the formula.
- Use the relationship amount in moles equals mass divided by molar mass.
- Give the unit mol with any calculated chemical amount.
- State that the relative formula mass Mr is found by adding the relative atomic masses of all atoms shown in the formula.
- Explain that the mass of one mole in grams is numerically equal to Mr, for example 44 g for CO₂ because Mr = 44.
- Use the relationship n = m ÷ Mr correctly, including rearranging to m = n × Mr when mass is required.
- Apply the idea to a range of substances, including simple molecules, ionic compounds and elements, using the correct formula for each.
- State that one mole of any substance contains the same number of stated particles.
- Identify the stated particle correctly, such as atoms in a metal, molecules in a covalent compound or ions in an ionic compound.
- Use the idea to compare amounts of different substances in moles, for example equal moles of two molecular gases contain equal numbers of molecules, or equal moles of any two substances contain equal numbers of their stated particles.
- Link the concept to balanced equations, where coefficients give the ratio of moles of each substance.
- State that the Avogadro constant is the number of particles in one mole of a substance.
- Recall the value 6.02 × 10²³ per mole and use it in calculations.
- Calculate the number of particles by multiplying the amount in moles by 6.02 × 10²³.
- Calculate the amount in moles by dividing the number of particles by 6.02 × 10²³.
- Explain that the mole can be applied to any stated particle, including atoms, molecules, ions and electrons.
- Compare equal amounts in moles of different substances and state that they contain the same number of stated particles.
- Use coefficients in a balanced equation to give the ratio of amounts in moles of reactants and products.
- Apply the mole concept to electrons, for example in electrolysis or redox, stating the particle as electrons.
- State the relationship amount in moles = mass ÷ relative formula mass and rearrange it to mass = moles × relative formula mass.
- Calculate Mr correctly by adding relative atomic masses for every atom shown in the formula, including brackets and any multipliers.
- Substitute values with consistent units, showing the division or multiplication before giving the answer.
- Give the final amount in mol or the final mass in g, with a sensible number of significant figures and correct unit.
- For a reverse calculation, identify the known quantity first, then choose the matching rearrangement rather than dividing when multiplication is required.
Examiner Tips
- 💡Write mol after every amount to secure the unit mark.
- 💡Show the molar mass calculation before substituting into the moles equation.
- 💡Check that mass is in grams and molar mass is in g/mol before dividing.
- 💡Always show the calculation of Mr step by step so the examiner can follow your reasoning.
- 💡Include units throughout, using g for mass, g mol⁻¹ for molar mass and mol for amount.
- 💡Check whether the question asks for mass, moles or Mr, and rearrange the equation before substituting numbers.
- 💡Always name the particle you are counting, for example atoms, molecules or ions.
- 💡Use the phrase 'same number of particles' when comparing equal amounts in moles.
- 💡When using equations, check that the coefficients are balanced before interpreting them as mole ratios.
- 💡Write the Avogadro constant in standard form with the correct exponent.
- 💡Show the calculation clearly, including whether you are multiplying or dividing by 6.02 × 10²³.
- 💡Give the particle type in your final answer, for example 'molecules of CO₂'.
- 💡Always state which particle the mole refers to in your answer.
- 💡When using an equation, write the mole ratio underneath the balanced equation before calculating.
- 💡For electron calculations, remember that one mole of electrons is 6.02 × 10²³ electrons.
- 💡Write the formula triangle or equation n = m ÷ Mr at the start so the rearrangement is visible to you and to the examiner.
- 💡Check whether the substance is a monatomic element (using Ar) or a compound/molecular element (using Mr), and show the addition that gives Mr.
- 💡Estimate first: 88 g of CO₂ should be about 2 mol because Mr is about 44, so a wildly different answer signals an error.
Common Mistakes
- Writing m as the symbol for mole; correct this by writing mol.
- Using relative atomic mass as a mass in grams without converting to molar mass; correct this by expressing molar mass in g/mol.
- Forgetting to state the type of particle when using the Avogadro number; correct this by naming the particles, such as atoms or molecules.
- Adding atomic masses incorrectly, such as forgetting to multiply by the subscript in CO₂; correct by multiplying each element's Ar by its subscript before adding.
- Confusing relative formula mass with the mass of one molecule in grams; correct by stating that Mr is a relative value and the molar mass is that number in grams per mole.
- Using the wrong rearrangement of n = m ÷ Mr; correct by checking units and rearranging carefully, for example m = n × Mr.
- Assuming one mole of any substance has the same mass; correct by stating that the number of particles is the same but the mass depends on the relative formula mass.
- Mixing up the type of particle, such as saying one mole of oxygen gas contains one mole of atoms rather than one mole of O₂ molecules; correct by naming the particle.
- Thinking that one mole of an ionic compound contains one mole of ions; correct by stating that it contains one mole of formula units, which contain the stated ions in the ratio shown by the formula.
- Writing the Avogadro constant without the correct power of ten, such as 6.02 × 10²²; correct by using 6.02 × 10²³.
- Forgetting to state the particle type when giving the number of particles; correct by naming atoms, molecules, ions or formula units.
- Multiplying by the Avogadro constant when converting from particles to moles; correct by dividing the number of particles by 6.02 × 10²³.
- Treating one mole of carbon dioxide as containing one mole of atoms; correct by stating it contains one mole of CO₂ molecules, which contain three moles of atoms in total.
- Ignoring the coefficients in an equation when comparing moles; correct by using the balanced coefficients as the mole ratio.
- Using the mole only for molecules and not for atoms, ions or electrons; correct by applying it to any stated particle.
- Using relative atomic mass of the first element only for a compound; correct by summing every atom, for example Mr of H₂O is 18, not 1 or 16.
- Multiplying when the question asks for moles from mass; correct by dividing mass by Mr for that direction.
- Omitting units or writing mol g⁻¹; correct by giving mol for amount and g for mass, remembering that Mr is dimensionless.