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AQA Chemistry A-level: Amount of Substance

Apr 23, 2025

AQA Chemistry A-level: Amount of Substance (3.1.2)

3.1.2.1 - Mr and Ar

  • Relative Atomic Mass (Ar):
    • Defined as the mean mass of an atom of an element divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.
  • Relative Molecular Mass (Mr):
    • Mean mass of a molecule of a compound, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.
    • Known as relative formula mass for ionic compounds.

3.1.2.2 - Moles and the Avogadro Constant

  • Mole:
    • Unit of measurement for substances, always contains the same number of particles.
  • Avogadro Constant (L):
    • Allows determination of the number of particles in a sample with known mass.
  • Importance:
    • Essential for various calculations, especially those involving concentration in mol/dm³.

3.1.2.3 - Ideal Gas Equation

  • Relationships:
    • Pressure is proportional to temperature.
    • Volume is proportional to temperature.
    • Pressure and volume are inversely proportional.
  • Ideal Gas Equation:
    • Combines these relationships.
    • Variables must be in standard units:
      • Pressure (p) in Pascals
      • Volume (V) in m³
      • Temperature (T) in Kelvin
      • Moles (n)
      • Mass (m) in grams
    • R: Ideal gas constant, 8.31 JK⁻¹ mol⁻¹.

3.1.2.4 - Empirical and Molecular Formula

  • Empirical Formula:
    • Simplest whole number ratio of atoms of each element in a compound.
    • Found using molar ratios.
  • Molecular Formula:
    • True number of each atom in the molecule.
    • Determined using Mr of the empirical formula and true Mr of the molecule.

3.1.2.5 - Equations and Calculations

  • Balancing Chemical Equations:
    • Essential for correct reacting ratios.
    • Used for calculating reacting masses, percentage yield, and atom economy.
  • Percentage Yield & Atom Economy:
    • High atom economy is desirable in industrial processes:
      • Minimizes waste products.
      • Economically viable and environmentally friendly.
      • Reduces use of natural resources and energy.