mole

Meaning: One mole is the amount of a substance that contains Avogadro's number, NA = 6.022 × 10²³, of specified particles. The same number of particles is found in 12 g of carbon-12 on the older definition, which is why molar mass in g/mol matches atomic or molecular mass in amu. Moles let you count particles by weighing them, and they are the common currency for stoichiometry, gas laws, concentration and equilibrium.

Equation: n = m/M, where n is the amount in mol, m is the mass in kg (grams are commonly used in chemistry), and M is the molar mass in kg/mol (g/mol is commonly used). Also N = n × NA, where N is the number of particles (dimensionless) and NA is in mol⁻¹.

On the MCAT: Expect to convert grams to moles to particles, find a limiting reagent from mole ratios in a balanced equation, compute molarity (mol/L) or use PV = nRT, or find empirical formulas from percent composition. Coefficients in a balanced equation give mole ratios, not mass ratios.

Common confusion: Students compare masses instead of moles. One mole of H₂ (about 2 g) and one mole of O₂ (about 32 g) contain the same number of molecules, but not the same mass. Also, a mole of a compound holds more than one mole of atoms: 1 mol of H₂O has 2 mol of H atoms and 1 mol of O atoms.