Atoms, Molecules, and Stoichiometry Flashcards

Relative Masses of Atoms and Molecules

• Relative Atomic Mass (Ar):

  • Mass of an atom relative to 1/12th of a carbon-12 atom.

  • Unified atomic mass: 1/12th of carbon-12 isotope ($1 u = 1.66 \times 10^{-27} kg$).

  • Ar is a ratio, so it has no units.

• Relative Isotopic Mass:

  • Isotopes: Atoms of the same element with different neutron numbers.

  • Relative isotopic mass: Mass of an isotope relative to 1/12th of carbon-12.

  • Average atomic mass calculation: Weighted average of isotopes.

  • $(\text{atomic mass of isotope} \times \text{percentage abundance})/100$

• Relative Molecular Mass (Mr):

  • Mass of a molecule relative to 1/12th of a carbon-12 atom.

  • Mr has no units.

  • Calculated by summing the Ar of all atoms in the molecule (or formula unit).

• Relative Formula Mass (Mr):

  • Used for ionic compounds.

  • Calculated the same way as relative molecular mass.

The Mole and Avogadro Constant

• Avogadro Constant (NA or L):

  • Number of particles equivalent to the relative atomic or molecular mass of a substance.

  • Applies to atoms, molecules, ions, and electrons.

  • $NA = 6.02 \times 10^{23} mol^{-1}$

• Mole (mol):

  • The mass of a substance containing NA particles.

  • Contains the same number of fundamental units as there are atoms in exactly 12.00 g of 12C.

Formulae

• Charges of ionic compounds

• Empirical Formula:

  • Simplest whole number ratio of elements in a compound.

  • Calculated from the ratio of masses of each element.

• Molecular Formula:

  • Shows the number and type of each atom in a molecule.

  • $(\text{relative formula mass of molecular formula}) / (\text{relative formula mass of empirical formula})$

Reacting Masses and Volumes

• Water of Crystallisation:

  • Water molecules part of a crystal structure in some compounds.

  • Hydrated compound: Contains water of crystallisation (e.g., $CuSO<em>4 \cdot 5H</em>2O$).

  • Anhydrous compound: Does not contain water of crystallisation (e.g., $CuSO_4$).

  • Conversion between anhydrous and hydrated forms is reversible by heating:

    • $CuSO<em>4 + 5H</em>2O \rightarrow CuSO<em>4 \cdot 5H</em>2O$

    • $CuSO<em>4 \cdot 5H</em>2O \rightarrow CuSO<em>4 + 5H</em>2O$