Add up the atoms, weighted by how many there are
Molar mass is the mass of one mole of a substance. To get it, take each element in the formula, multiply its atomic weight by its subscript, and total the results. The subscripts are the whole point — treating H₂O as one H plus one O is the most common mistake.
Worked example — glucose (C₆H₁₂O₆)
| Element | Atomic weight | Count | Subtotal |
|---|---|---|---|
| Carbon (C) | 12.011 | 6 | 72.07 |
| Hydrogen (H) | 1.008 | 12 | 12.10 |
| Oxygen (O) | 15.999 | 6 | 95.99 |
| Total | 180.16 g/mol |
Keep full precision on the atomic weights and round only the final answer; rounding early can shift the result by a few tenths.
Molar masses of common compounds
| Compound | Formula | Molar mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.02 |
| Table salt | NaCl | 58.44 |
| Carbon dioxide | CO₂ | 44.01 |
| Ethanol | C₂H₆O | 46.07 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Sulfuric acid | H₂SO₄ | 98.08 |
| Caffeine | C₈H₁₀N₄O₂ | 194.19 |
What molar mass lets you do
- Grams to moles. n = m ÷ M. Divide a weighed mass by the molar mass to get moles.
- Moles to grams. m = n × M. To weigh out 2.5 mol of glucose: 2.5 × 180.16 = 450.4 g.
- Parentheses. Distribute the outside subscript first: Ca(OH)₂ has 2 O and 2 H, giving 74.09 g/mol.
Common questions
How do you calculate the molar mass of a compound?
Multiply each element's atomic weight by how many of that atom the formula contains, then add the pieces up. For water, H2O: 2 times 1.008 plus 15.999 equals 18.02 g/mol.
Is molar mass the same as molecular weight?
Numerically yes, but the units differ. Molecular weight is the mass of one molecule in atomic mass units; molar mass is the mass of one mole (6.022 x 10^23 molecules) in grams per mole. The figures match, so most people use them interchangeably.
Do I include the water in a hydrate like CuSO4 5H2O?
Yes. The waters of crystallisation are part of the formula. CuSO4 is 159.6 g/mol, but CuSO4 5H2O is 249.7 g/mol, and using the wrong one throws off every weighing.
