one mole
Summary
One mole is the amount of a substance containing 6.022 × 10²³ elementary entities (atoms, molecules, ions, or formula units), a number called Avogadro's number. It is a count, not a mass: equal moles mean equal numbers of particles, and molar mass (g/mol) converts between grams and moles.
Detail
Meaning: A mole is a counting unit, like a dozen, for 6.022 × 10²³ particles (Avogadro's number). An element's atomic mass in amu equals its molar mass in g/mol, so a mole of carbon-12 weighs 12 g. Balanced-equation coefficients give mole ratios, not mass ratios.
Equation: n = m/M and N = nNA, where n is amount (mol), m is mass (g; kg in strict SI), M is molar mass (g/mol; kg/mol in strict SI), N is the number of particles (dimensionless), and NA is the Avogadro constant, 6.022 × 10²³ mol⁻¹.
On the MCAT: Moles appear in most quantitative passages: grams to moles, limiting reagent, percent yield, molarity (mol/L), and n in PV = nRT. For equal masses, the smaller molar mass gives more moles (10 g of He has more atoms than 10 g of Ne).
Common confusion: Students treat a mole as a mass. One mole of different substances has the same number of particles but different masses. Also count the right particles: one mole of H₂O contains 2 mol of H atoms, and one mole of CaCl₂ gives about 3 mol of ions when dissolved.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.