molar mass

Meaning: Molar mass is the mass of one mole (6.02 × 10²³ particles) of an element or compound. For an element, it is the atomic mass on the periodic table, expressed in g/mol. For a compound, add the atomic masses of every atom in the formula, using the subscripts. The number is the same as the mass of one particle in amu, so one table serves both scales.

Equation: n = m/M, where n is the amount of substance in mol, m is the sample mass in g (kg in strict SI), and M is the molar mass in g/mol (kg/mol in strict SI, though g/mol is used in practice).

On the MCAT: Molar mass is the first step in most quantitative problems. Examples are stoichiometry, limiting reagent and percent yield, empirical formulas from percent composition, and molarity from a mass of solute. It also appears in gas questions: n = m/M in PV = nRT, density d = PM/RT, and Graham's law, where effusion rate is proportional to 1/√M, so a heavier gas effuses more slowly.

Common confusion: Molar mass is not the mass number. The mass number is a whole-number count of protons plus neutrons in one isotope. Molar mass is a weighted average over natural isotopes, so it is rarely a whole number. Also, count every atom inside parentheses: Ca(OH)₂ has two O and two H. For ionic compounds, "molar mass" means the mass of one mole of formula units.