The mole and the Avogadro constant — AQA A-Level Chemistry
Test yourself on The mole and the Avogadro constant with AQA A-Level practice questions.
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The mole and the Avogadro constant explained
A mole is a counted amount: it contains as many particles as the Avogadro constant, 6.022 × 10²³ mol⁻¹.
Read the full explanation
The link is number of particles = amount in moles × the Avogadro constant, which rearranges to give the amount from a count. It also converts between the scale of a single particle and the scale of the laboratory: the mass of one atom in grams multiplied by the Avogadro constant gives the mass of one mole, which is how a time of flight calculation turns a measured ion mass of 1.396 × 10⁻²² g into a molar mass of 84.0 g mol⁻¹. State which particle you are counting, because one mole of calcium chloride contains one mole of calcium ions but two moles of chloride ions.
The mole as applied to electrons, atoms, molecules, ions, formulas and equations.
The mole is the bridge between what you can weigh and what the equation counts. Three routes give an amount: n = mass ÷ molar mass for a solid, n = concentration × volume in dm³ for a solution, and n = pV/RT for a gas. Once you have an amount, the coefficients in the balanced equation give the ratio to any other species, including electrons in a half-equation, where the number in front of the electron sets how many moles of charge flow per mole of reactant. Amounts also scale within a formula: one mole of sulfuric acid supplies two moles of hydrogen ions. Every multi-step calculation follows the same shape: get to moles, use the ratio, convert back.
Your focus
- Calculate the number of ions of a named type in a given mass of an ionic compound.
- Convert the mass of a single atom in grams into a molar mass using the Avogadro constant.
- Explain why one mole of calcium chloride does not contain the same number of positive and negative ions.
Show all 6 objectives
- Calculate the amount in moles of a substance from its mass, or from the concentration and volume of its solution.
- Use the coefficients of a balanced equation to convert an amount of one species into an amount of another.
- Determine the amount of electrons transferred per mole of reactant from a given half-equation.
The mole and the Avogadro constant exam tips
Marking Points
- one mark for number of particles obtained as amount in moles multiplied by the Avogadro constant
- one mark for converting a mass to an amount in moles before counting particles
- one mark for scaling by the number of that particle in one formula unit
- one mark for naming the particle counted, such as ions, atoms or molecules
- one mark for an amount calculated as mass divided by molar mass
- one mark for an amount in solution calculated as concentration multiplied by volume in cubic decimetres
- one mark for applying the stoichiometric ratio taken from the balanced equation
- one mark for converting the final amount back into the quantity the question asks for
Examiner Tips
- 💡Write down what is being counted before you calculate; the word ions or atoms in the question changes the answer.
- 💡Check the direction of the conversion by asking whether the answer should be a huge number or a small one.
- 💡Carry the units through: mol multiplied by mol⁻¹ leaves a pure number of particles.
- 💡Write the balanced equation at the top of the working and ring the two species you are converting between.
- 💡Label every amount with its species, for example n(hydrochloric acid), so ratios cannot be applied to the wrong substance.
- 💡Keep full calculator precision until the final line, then round.
Common Mistakes
- dividing by the Avogadro constant when the number of particles is required
- leaving a mass in kilograms when the molar mass is in grams per mole
- counting formula units when the question asks for ions, so ignoring the subscripts
- treating the Avogadro constant as a mass rather than a count per mole
- using a volume in cubic centimetres directly with a concentration in mol dm⁻³
- inverting the mole ratio so the answer is out by the stoichiometric factor
- using the relative atomic mass of an element when the substance is diatomic, such as using 16.0 for oxygen gas
- forgetting that one mole of a compound supplies more than one mole of some of its ions