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Amount of substance
This topic develops quantitative chemistry using moles, concentrations, equations, atom economy and yield.
30
Objectives
10
Flashcards
10
Questions
90 min
Study time
AqaA LevelChemistryPhysical chemistry
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Syllabus checklist
What you need to know
30 objective pages available
Relative atomic mass and relative molecular mass4 objectives
- Define relative atomic mass in relation to carbon-12.
- Define relative molecular mass in relation to carbon-12.
- Use relative formula mass for ionic compounds.
- Calculate relative molecular or formula mass from a chemical formula and relative atomic masses.
The mole and the Avogadro constant6 objectives
- Explain the Avogadro constant as the number of particles in one mole.
- Apply the mole to electrons, atoms, molecules, ions, formula units and equations.
- Calculate amounts using particle number and the Avogadro constant.
- Calculate amounts using mass, Mr and moles.
- Calculate amounts using concentration, volume and moles in solution.
- Use standard form, appropriate units and significant figures in mole calculations.
The ideal gas equation4 objectives
- Use pV = nRT in calculations involving gases.
- Rearrange the ideal gas equation to calculate pressure, volume, amount, gas constant or temperature where appropriate.
- Convert values into SI units before using the ideal gas equation.
- Explain why unit consistency matters in ideal gas calculations.
Empirical and molecular formula5 objectives
- Calculate empirical formulae from percentage composition data.
- Calculate empirical formulae from experimental mass data.
- Derive molecular formulae from empirical formulae and relative molecular mass.
- Use combustion analysis data to determine empirical formulae.
- Distinguish empirical formula from molecular formula.
Balanced equations and reacting quantities6 objectives
- Write balanced equations for reactions studied in the specification.
- Use balanced equations to calculate reacting masses.
- Use balanced equations to identify limiting reagents.
- Calculate percentage yield from actual and theoretical yield.
- Calculate atom economy for a reaction.
- Explain why percentage yield and atom economy are different measures.
Titration calculations5 objectives
- Use titration results to calculate unknown concentrations.
- Use mean titre values appropriately in calculations.
- Prepare and use standard solutions in quantitative analysis.
- Link acid-base stoichiometry to balanced equations.
- Required practical: make up a volumetric solution and carry out a simple acid-base titration.
Key terms
Relative atomic massIsotopic massRelative molecular massMolar massStoichiometryAvogadro constantMoleAvogadro’s constantFormula unitAvogadro's constantMolar MassConcentration
Exam tips
- Show the weighted‑average formula clearly: When calculating a relative atomic mass, write out the full expression Σ(m_i × a_i) before substituting numbers.
- Always specify the reference for relative masses.: When defining relative atomic or molecular mass, always mention that it is in relation to carbon-12.
Common mistakes
- Confusing relative atomic mass with atomic weight: Remember that relative atomic mass is a weighted average of isotopic masses and is usually expressed to two or more decimal places.
- Confusing relative atomic mass with relative molecular mass: Remember that relative atomic mass refers to individual atoms, while relative molecular mass refers to molecules.
Practice preview
- Which statement best defines the relative atomic mass of an element?
- Explain how the relative atomic mass of an element is determined from its naturally occurring isotopes.
- Chlorine has two naturally occurring isotopes: ^35Cl (mass 34.96885 u, abundance 75.77 %) and ^37Cl (mass 36.96590 u, abundance 24.23 %). Calculate the relative atomic mass of chlorine to four significant figures.
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