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Amount of substance
This topic develops quantitative chemistry using moles, concentrations, equations, atom economy and yield.
30
Objectives
50
Flashcards
36
Questions
90 min
Study time
AQAA LevelChemistryPhysical chemistry
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Syllabus checklist
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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 masscarbon-12relative molecular massrelative formula massionic compoundAvogadro constantmoleparticle numberrelative molecular mass (Mr)concentrationmolesIdeal Gas Equation
Exam tips
- Understanding Relative Atomic Mass: Remember that relative atomic mass (Ar) is defined as the weighted average mass of an atom of an element compared to 1/12th of the mass of a carbon-12 atom.
- Understanding Relative Molecular Mass: To define relative molecular mass (Mr), remember it is the mass of one mole of a substance compared to carbon-12. Use the formula: Mr = sum(Ar of all atoms in the formula).
Common mistakes
- Confusing Relative Atomic Mass with Mass Number: Remember that relative atomic mass is the weighted average mass of an atom compared to carbon-12, while mass number is the total number of protons and neutrons in an atom. To define relative atomic mass in relation to carbon-12, use the formula: Ar = (mass of atom / mass of carbon-12) x 12. For example, if the mass of an atom is 24, then Ar = (24 / 12) x 12 = 24. Therefore, the relative atomic mass is 24.
- Confusing Relative Molecular Mass with Atomic Mass: To fix this, remember that relative molecular mass (Mr) is the sum of the relative atomic masses of all atoms in a molecule. For example, for water (H2O), calculate Mr as follows: Mr = (2 × Ar of H) + (1 × Ar of O) = (2 × 1) + (16) = 18. Therefore, the relative molecular mass of water is 18 g/mol.
Practice preview
- Calculate the relative atomic mass of magnesium (Mg) given that it has 12 protons and 12 neutrons.
- Calculate the relative molecular mass of water (H2O) using the relative atomic masses of hydrogen (H = 1) and oxygen (O = 16).
- Calculate the relative formula mass of sodium chloride (NaCl) using the relative atomic masses of sodium (Na = 23) and chlorine (Cl = 35.5).
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