Formulae, FW's, and MW's:

Formula weights, atomic weights and molecular weights may all be expressed in two distinct ways:

1. amu's/particle for a single atom, molecule or "formula unit," or
2. they may be expressed in grams/mole for macroscopic quantities.

Recall also some important terms for describing substances:

• Empirical Formula: the smallest whole number ratio of atoms in a compound.
• Formula weight (FW): the sum of the atomic weights of the atoms in a formula. Note that no specific particle is implied. A formula weight could correspond to a molecule, or it could correspond to a crystal of ions, or it could correspond to an association of different particles such as water molecules hydrating ions or molecules etc., or  it could correspond to the smallest repeating unit in a polymer.
• Molecular weight: the sum of the atomic weights in a single molecule of covalently bonded atoms.

© R A Paselk

Last modified 16 October 2008