Avogadro's constant

Meaning: Avogadro's constant is the number of elementary particles (atoms, molecules, ions or formula units) in one mole, about 6.022 × 10²³ mol⁻¹. It links the atomic scale to the lab scale. A particle's mass in amu is numerically equal to the mass of one mole of those particles in grams, so molar mass in g/mol lets you move between grams, moles and particle counts.

Equation: N = n × NA, where N is the number of particles (a pure count, no unit), n is the amount of substance in moles (mol), and NA is Avogadro's constant (mol⁻¹). For one particle, mass = M/NA, where M is molar mass (kg/mol in SI units, though g/mol is usual in problems).

On the MCAT: Expect a conversion chain such as grams → moles (divide by molar mass) → particles (multiply by NA), often asking for atoms of one element in a compound. It also appears in gas problems, where n = PV/RT is found first, and in electrochemistry, where the charge on one mole of electrons is NA × e (the Faraday constant, about 9.65 × 10⁴ C/mol). Estimating with powers of ten is usually enough.

Common confusion: Avogadro's constant is not Avogadro's law. The law says equal volumes of ideal gases at the same T and P contain equal numbers of particles. Also, NA counts whatever particle you name: one mole of H₂O is NA molecules but 3NA atoms.