The definition
The molar mass M of a chemical species is the mass of one mole of it. It’s expressed in g/mol and found by adding up the atomic molar masses of every atom in its molecular formula.
The 3-step method
- Write the molecular formula, for example C2H6O for ethanol.
- Multiply each element’s atomic molar mass by its number of atoms.
- Add everything up.
Useful atomic molar masses
| Element | M (g/mol) |
|---|---|
| H (hydrogen) | 1.008 |
| C (carbon) | 12.011 |
| N (nitrogen) | 14.007 |
| O (oxygen) | 15.999 |
| Na (sodium) | 22.990 |
| S (sulfur) | 32.06 |
| Cl (chlorine) | 35.45 |
IUPAC standard values, rounded.
Example 1: water, H2O
M = 2 × 1.008 + 15.999 = 18.015 g/mol
Example 2: ethanol, C2H6O
M = 2 × 12.011 + 6 × 1.008 + 15.999 = 24.022 + 6.048 + 15.999 = 46.069 g/mol
Example 3: caffeine, C8H10N4O2
M = 8 × 12.011 + 10 × 1.008 + 4 × 14.007 + 2 × 15.999
= 96.088 + 10.080 + 56.028 + 31.998 = 194.194 g/mol, about 194.19 g/mol.
Common mistakes
- Forgetting implicit hydrogens when starting from a skeletal formula.
- Mixing up molar mass (g/mol) and mass (g): they’re linked by n = m / M.
- Rounding too early: keep the decimals until the final result.
- Forgetting the water in hydrated salts, such as CuSO4·5H2O.
Automatic calculation
In the Ejja Chem editor, draw or search the molecule: its molecular formula and molar mass appear live in the status bar. Try it with caffeine →