1.1 Moles and Molar Mass

The mole as a counting unit

A mole is a fixed amount of substance equal to 6.022 × 10²³ particles (Avogadro's number), the same way "a dozen" always means 12. Chemists use moles because atoms and molecules are too small to count individually, but moles let us relate microscopic particles to measurable macroscopic quantities like grams.

Molar mass connects grams and moles

Molar mass is the mass in grams of one mole of a substance (g/mol), numerically equal to the atomic/molecular mass in amu found on the periodic table. It acts as a conversion factor: moles = mass (g) ÷ molar mass (g/mol). For compounds, add the molar masses of all atoms in the formula.

Why moles matter in chemistry

Moles allow direct comparison of the number of particles between substances, which is essential for stoichiometry, solution concentration, and gas laws. Because reactions occur particle-to-particle, converting mass into moles lets chemists predict how much product forms or how much reactant is needed.

Key terms

Mole

A unit representing 6.022 × 10²³ particles of a substance (Avogadro's number).

Avogadro's number

6.022 × 10²³ particles per mole.

Molar mass

The mass in grams of one mole of a substance, in g/mol; numerically equal to atomic/molecular mass in amu.

Moles formula

moles = mass (g) ÷ molar mass (g/mol).

Why moles are useful

They connect the countable number of particles to measurable mass, essential for stoichiometry and lab calculations.

Try an AP-style question

How many moles are in 36.0 g of water (H₂O, 18.0 g/mol)?

1. 0.500 mol
2. 1.00 mol
3. 2.00 mol
4. 648 mol