Introduction
Chemists can't count atoms one by one — they're far too small and far too numerous. Instead, chemists use a special counting unit called the mole, which lets us connect the invisible world of atoms and molecules to things we can measure, like mass and volume. This lesson introduces the amount of substance and the key formulas you'll use throughout the stoichiometry unit.
The mole and Avogadro's constant
The mole (mol) is the SI unit for amount of substance, symbol n. One mole of any substance contains exactly the same number of particles: 6.02 × 10²³, known as the Avogadro constant (N_A).
- 1 mole of carbon atoms = 6.02 × 10²³ carbon atoms
- 1 mole of H₂O molecules = 6.02 × 10²³ H₂O molecules
- 1 mole of Na⁺ ions = 6.02 × 10²³ Na⁺ ions
Molar mass
The molar mass (M), measured in g mol⁻¹, is the mass of one mole of a substance. You find it by adding up the relative atomic masses (from the periodic table) of all atoms in the formula.
Worked Example 1 Find the molar mass of Na₂SO₄·10H₂O.
M = (2 × 22.99) + 32.07 + (4 × 16.00) + 10 × [(2 × 1.01) + 16.00]
M = 45.98 + 32.07 + 64.00 + 180.00
M ≈ 322 g mol⁻¹
(Talbot, p. 96)
The key formula: n = m / M
Once you know the molar mass, you can convert between mass and amount using:
n = m / M
where n = amount (mol), m = mass (g), M = molar mass (g mol⁻¹).
Worked Example 2 How many moles of mercury(II) oxide are present in 1.25 g of H