Molar MassMolar mass, molecular mass and formula mass
Textbooks use several names that give the same number. That causes more confusion than the calculation itself. Here is what each one means.
| Term | What it is | Unit | For water |
|---|---|---|---|
| Molecular mass | Mass of one molecule | u (unified atomic mass unit) | 18.015 u |
| Relative molecular mass, Mr | Mass of one molecule compared with one twelfth of a carbon-12 atom | none | 18.015 |
| Formula mass | The same idea for a substance without molecules, such as a salt | u | not used |
| Molar mass, M | Mass of one mole | g/mol | 18.015 g/mol |
| Gram molecular mass | Older name: the molecular mass expressed in grams | g | 18.015 g |
| Molecular weight | Older name for relative molecular mass, still common in biology and industry | none | 18.015 |
One particle or one mole
The real difference is scale. Molecular mass is about a single molecule and is measured in a unit small enough for atoms, the unified atomic mass unit, u, also called the dalton, Da. Molar mass is about a mole of molecules and is measured in grams per mole. The numbers match by design: the mole was chosen so that a substance whose molecules have a mass of 18 u has a molar mass of 18 g/mol.
Why "formula mass" exists
Sodium chloride is not made of NaCl molecules. It is a giant lattice of sodium ions and chloride ions. So there is no molecule to have a molecular mass. Chemists speak instead of the formula unit, NaCl, and its formula mass, 58.44 u. The molar mass, 58.44 g/mol, is used in exactly the same way as for a molecular substance. The same applies to silicon dioxide, metals and all salts.
Relative means no unit
Relative atomic mass and relative molecular mass are ratios, so they have no unit. A water molecule is 18.02 times as heavy as one twelfth of a carbon-12 atom. Exam boards that use the symbols Ar and Mr expect answers without units for these, and with g/mol for molar mass.
A small point since 2019
Before 2019, the molar mass in g/mol was exactly equal in number to the relative molecular mass. Since the redefinition of the mole, the link between them, called the molar mass constant, is no longer exactly 1 g/mol but so close to it that the difference appears only around the tenth significant figure. No school, college or ordinary laboratory calculation is affected.
Gram atom and gram molecule
Older Indian textbooks use gram atom and gram molecule. One gram atom of an element is its atomic mass in grams, that is, one mole of its atoms. One gram molecule is one mole of molecules. "2 gram molecules of water" simply means 2 moles, 36.03 g.
Which word should you use?
- Talking about one molecule: molecular mass, in u.
- Talking about a salt or a network solid: formula mass.
- Doing any calculation with grams and moles: molar mass, in g/mol.
Whatever the name in the question, the working is the same, and it is set out on the step by step page.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.