The Mole and Molar Mass

The mole is a crucial counting unit in chemistry. It represents the amount of a substance that contains exactly as many elementary particles as there are atoms in 12 grams of Carbon-12. This specific number is called Avogadro's number, which equals 6.022×10²³.

Converting between moles and number of particles is straightforward. For example, 3.00 mol of calcium contains 1.807×10²⁵ calcium atoms $3.00 mol × 6.022×10²³ atoms/mol$. Working backward, 1.00×10²⁰ calcium atoms equals 1.00×10⁻⁴ moles (1.00×10²⁰ ÷ 6.022×10²³).

Molar mass is the mass in grams of one mole of a substance. For instance, carbon has a molar mass of 12.01 g/mol, so 25.00 g of carbon equals 2.082 moles $25.00 g ÷ 12.01 g/mol$.

Quick Tip: Remember that you can easily convert between grams, moles, and number of atoms using this relationship: grams ↔ moles ↔ atoms. Divide by molar mass to convert from grams to moles, or multiply by molar mass to convert from moles to grams.