Recall that formula weights, atomic weights and molecular weights may all be expressed in two distinct ways:
- amu's/particle for a single atom, molecule or "formula unit," or
- 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.
Moles and Molarity
When talking about atoms, molecules, etc. we are talking about individual entities, so we need a unit of amount of substance in the sense of a counted amount. The unit of substance is the mole.
Whenever we do problems involving formulae or chemical equations, in other words ratios of particles, we need to use moles. (Chemists frequently use a special term for such expressions involving ratios: Stoichiometry. We will spend some time on this concept later [see Burns, pg 315].)
So what is a mole? It is the number of atoms in 12 grams of 12C. This turns out to be a huge number: 6.022 x 1023, which is given the name Avogadro's Number. Obviously it is a constant.
Note that when we look at the Periodic Chart the atomic masses have two meanings for chemists and for us:
- atomic mass = mass in atomic mass units for one average atom of that element = amu/atom;
- atomic mass = mass in grams for one mole of that element = g/mol.
For example, for the equation:
2H2 + O2 2 H2O
We have two meanings:
- Two molecules of hydrogen combine with one molecule of oxygen to give two molecules of water; thus 2 (2 amu) + 32 amu = 2 (18 amu), or 4 amu + 32 amu = 36 amu
- Two moles of hydrogen combine with one mole of hydrogen to give two moles of water; thus 2 (2 g) + 32 g = 2 (18 g), or 4 g + 32 g = 36 g.
2Al + 3S
- Two atoms of aluminum combine with three atoms of sulfur to give one "formula unit" of aluminum sulfide; thus 2(27.0 amu) + 3(32.1 amu) = 150.3 amu, or 54 amu + 96.3 amu = 150.3 amu.
- Two moles of aluminum combine with three moles of sulfur to give one mole of aluminum sulfide; thus 2(27.0 g) + 3(32.1 g) = 150.3 g, or 54 g + 96.3 g = 150.3 g.
