Molar Mass Calculator (Molecular Weight from Formula)
Type a chemical formula to get its molar mass and percent composition.
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Molar Mass Calculator (Molecular Weight from Formula) by TheCalcTools
How the Molar Mass Calculator (Molecular Weight from Formula) works
The molar mass is the sum of the atomic weights of every atom in the formula, in grams per mole. Numbers after a symbol count atoms; numbers after a bracket multiply everything inside it; a number at the start of a hydrate part multiplies that part.
Atomic weights are the IUPAC conventional values (for example H 1.008, C 12.011, O 15.999, Cl 35.45). For elements with no stable isotopes the mass number of the longest-lived isotope is used.
The formula
Worked example
Using the default values shown in the calculator:
| Chemical formula | C6H12O6 |
|---|
- Glucose, C6H12O6: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol.
- Mass percent: C 40.00%, H 6.71%, O 53.28%.
Result:
| Molar mass (g/mol) | 180.156 |
|---|---|
| Mass percent of each element | C 40.00%, H 6.71%, O 53.28% |
| Atoms of each element | C 6, H 12, O 6 |
| Atoms per formula unit | 24 |
| Mass of one molecule (g) | 2.992e-22 g |
Frequently asked questions
Is molar mass the same as molecular weight?
Numerically yes. Molar mass has units of g/mol; molecular weight (relative molecular mass) is the same number without units.
How do I enter a hydrate?
Write the water part after a dot: CuSO4.5H2O or CuSO4·5H2O. The 5 multiplies H2O.
Why does my textbook give a slightly different value?
Textbooks round atomic weights differently (for example Cl 35.5 or 35.45). Differences in the second decimal place are normal.
Sources and further reading
- IUPAC CIAAW: Standard atomic weights
- Wikipedia: Molar mass
- periodictable (Python package, atomic weight data)
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