Amount of Substance — A-Level Chemistry Revision

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What is Amount of Substance?

This topic is the bedrock of quantitative chemistry, focusing on the mole as the unit for amount of substance. It involves calculations using Avogadro's constant, molar mass, gas volumes, and solution concentrations. Mastering these calculations is crucial for determining reacting masses, percentage yields, and empirical or molecular formulae from experimental data.

Board notes: All boards (AQA, Edexcel, OCR) place a heavy emphasis on mole calculations as they are fundamental to all other chemistry topics. AQA often features multi-step calculations involving gas laws and titrations. Edexcel may include more context-based problems, such as industrial processes. OCR frequently tests understanding of atom economy and percentage yield in their questions.

Worked example

Common mistakes

• 1Using mass instead of moles in stoichiometric ratios. Chemical equations relate the molar quantities of reactants and products, not their masses.
• 2Forgetting to convert units, especially from cm3 to dm3 for solution concentration calculations (divide by 1000), or from Celsius to Kelvin for the ideal gas equation (add 273).
• 3Confusing empirical formula with molecular formula. The empirical formula is the simplest whole-number ratio of atoms, while the molecular formula gives the actual number of atoms of each element in a molecule.

Amount of Substance exam questions

Exam-style questions for Amount of Substance with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the difference between relative atomic mass and molar mass?

  Relative atomic mass (Ar) is the weighted average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom. Molar mass (M) is the mass of one mole of a substance and has units of g/mol; its numerical value is the same as the relative formula mass.

• How do I use the ideal gas equation, pV=nRT?

  Ensure all your variables are in the correct SI units: pressure (p) in Pascals (Pa), volume (V) in cubic metres (m3), temperature (T) in Kelvin (K), and n is moles. The gas constant, R, is 8.31 J K-1 mol-1. Be careful with unit conversions, especially for pressure and volume.