Avogadro's number

Meaning: Avogadro's number is the number of elementary entities (atoms, molecules, ions, electrons) in one mole. It converts between single particles and measurable grams. Since 1 u ≈ 1.66 × 10⁻²⁴ g, a substance's mass in u per particle equals its molar mass in g/mol.

Equation: N = n NA, where N is the number of particles (unitless), n is the amount in moles (mol), and NA = 6.02 × 10²³ mol⁻¹. With n = m/M, m is mass (kg in SI, often g) and M is molar mass (kg/mol in SI, often g/mol).

On the MCAT: Expect chains such as grams → moles → particles, or counting ions from a formula: 1 mol of CaCl₂ gives 3 mol of ions, about 1.81 × 10²⁴ ions. It also appears in the Faraday constant, F = e NA ≈ 96,500 C/mol, which links charge to moles of electrons.

Common confusion: Avogadro's number is a count, not a mass or a volume. One mole of different substances has the same number of particles but different masses. Do not confuse it with molar volume (22.4 L/mol for an ideal gas at STP), and remember to multiply by the number of atoms per molecule when the question asks about atoms.