In textbooks and chemistry classrooms, the terms molecular weight, molar mass, and formula mass are often used interchangeably. However, to a chemical physicist or standards committee, they have distinct definitions.
1. Molecular Weight (or Molecular Mass)
Molecular weight is the mass of a single molecule of a substance. It is measured in unified atomic mass units (u) or Daltons (Da), where 1 u is defined as 1/12th the mass of a carbon-12 atom.
Because it is the mass of a single molecule, it refers strictly to covalent substances (like water, methane, or glucose) that exist as individual, distinct molecules.
2. Molar Mass
Molar mass is the mass of one mole (6.02214076 × 10²³) of a substance. It is expressed in grams per mole (g/mol).
While the numerical value of molar mass in g/mol matches the molecular weight in unified atomic mass units, their physical dimensions are different. Molecular weight measures *atomic-scale mass*, whereas molar mass measures *macroscopic-scale mass per mole*.
3. Formula Weight
Formula weight applies to substances that do not exist as isolated molecules, particularly ionic compounds. For example, Table Salt (NaCl) does not form separate NaCl molecules. Instead, it forms a continuous 3D crystalline lattice of Na⁺ and Cl⁻ ions.
For these structures, we sum the atomic weights according to the simplest empirical formula, producing the formula weight.