The mole is the fundamental counting unit of chemistry, defined as exactly 6.02214076 × 10²³ entities (Avogadro's number). This definition was formally adopted in 2019, replacing the older definition based on carbon-12. One mole of any substance contains the same number of particles — whether atoms, molecules, ions, or formula units.
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). For elements, the molar mass equals the atomic mass shown on the periodic table in atomic mass units (u or Da). For compounds, add the molar masses of all atoms in one formula unit: for H₂O, M = 2(1.008) + 15.999 = 18.015 g/mol.
The mole acts as a bridge between the macroscopic (laboratory) scale and the microscopic (atomic) scale. Three conversions connect these worlds:
For example, 36.03 g of H₂O contains 36.03 / 18.015 = 2.000 mol of water molecules, or 2.000 × 6.022 × 10²³ = 1.204 × 10²⁴ molecules.
Percent composition by mass is calculated as: % element = (mass of element in 1 mol of compound / molar mass of compound) × 100. This is tested directly on the AP Chemistry exam in MCQ format.
The empirical formula is the simplest whole-number ratio of atoms in a compound. To find it from percent composition: (1) assume 100 g of compound, (2) convert grams of each element to moles, (3) divide all mole amounts by the smallest, (4) round to whole numbers (or multiply to eliminate fractions). The molecular formula is a whole-number multiple of the empirical formula: molecular formula mass / empirical formula mass gives the multiplier.