Guides
How to Calculate Molar Mass — Formula, Steps & Examples
Molar mass is the mass of one mole of a substance, in g/mol, numerically equal to the relative molecular mass. The method is simple: multiply each element's relative atomic mass by its number of atoms in the formula, then add them all up.
Formula
M = Σ(number of atoms × relative atomic mass), in g/mol. Atomic masses come from IUPAC standard values (e.g. H≈1.008, C≈12.011, O≈15.999).
Steps
- Count the atoms Write the formula and count each element's atoms; remember subscripts apply to everything inside brackets, and water of crystallization (·nH₂O) counts too.
- Look up atomic masses Find the relative atomic mass of each element.
- Multiply For each element: number of atoms × relative atomic mass.
- Add up Sum the per-element subtotals to get the molar mass (g/mol).
Worked examples
| Substance | Calculation | Molar mass |
|---|---|---|
| H₂O | 2×1.008 + 16.00 | 18.02 g/mol |
| H₂SO₄ | 2×1.008 + 32.06 + 4×16.00 | 98.08 g/mol |
| Ca(OH)₂ | 40.08 + 2×(16.00+1.008) | 74.09 g/mol |
| CuSO₄·5H₂O | 63.55+32.06+4×16.00 + 5×18.02 | ≈ 249.7 g/mol |
Common mistakes
- A subscript after a bracket multiplies every atom inside, e.g. Ca(OH)₂ has 2 O and 2 H.
- Always include water of crystallization: CuSO₄·5H₂O adds 5 × H₂O.
- Use the element's relative atomic mass (O=16.00), not its atomic number (O is element 8).
Skip the arithmetic — type any formula into the molar mass calculator for M plus each element's mass fraction.