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.