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., CuSO4 \cdot 5H2O).
Anhydrous compound: Does not contain water of crystallisation (e.g., CuSO_4).
Conversion between anhydrous and hydrated forms is reversible by heating:
CuSO4 + 5H2O \rightarrow CuSO4 \cdot 5H2O
CuSO4 \cdot 5H2O \rightarrow CuSO4 + 5H2O