molar mass
/ MOH-lar mass /
You cannot count atoms one by one on a balance, but you can weigh them in convenient bulk. Molar mass is the weight of exactly one mole — a fixed, enormous count — of a substance, the conversion rate between grams you can measure and moles the chemistry cares about.
Molar mass is the mass per mole, with units of grams per mole (g/mol). Numerically it equals the formula or molecular weight, and you find it by summing the atomic masses of every atom in the formula and reading the answer in grams.
It is the single most-used conversion in quantitative chemistry: dividing a measured mass by molar mass gives moles, the currency of every reaction equation. Get the formula or the atomic masses slightly wrong and every downstream concentration and yield inherits the error.
Glucose (C₆H₁₂O₆) has a molar mass of about 180 g/mol, so weighing out 90 g gives 0.5 mol — half a mole's worth of sugar molecules.
Molar mass converts grams on the balance into moles.
Molar mass (g/mol) carries units; the closely related molecular and formula weights are sometimes quoted as plain numbers in atomic mass units. The figure is the same, but always track the units.