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Molar Mass & Molecular Weight Calculator
Type any chemical formula to get its molecular weight in g/mol, the mass and percentage of each element, and quick grams ⇄ moles conversion. Handles brackets and water of crystallisation, e.g. CuSO4·5H2O or K4[Fe(CN)6]·3H2O.
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How it works
Moles = Mass (g) ÷ Molar mass (g/mol)
Atomic weights are the IUPAC standard values (rounded conventional values, e.g. H 1.008, C 12.011, O 15.999, Cl 35.45). Results usually agree with reagent labels to within a few hundredths of a g/mol.
How to type formulas
- Start each element with a capital letter: NaCl, not nacl. CO is carbon monoxide; Co is cobalt.
- Use brackets for groups: Ca(OH)2, (NH4)2SO4, K4[Fe(CN)6].
- Write hydrates with a dot: CuSO4·5H2O, CuSO4.5H2O or CuSO4*5H2O.
Worked example
Copper sulphate pentahydrate, CuSO₄·5H₂O
Cu 63.546 + S 32.06 + O 4 × 15.999 + 5 × (2 × 1.008 + 15.999) = 249.68 g/mol. Anhydrous CuSO₄ is 159.60 g/mol, so if a method specifies the anhydrous salt but you have the pentahydrate, weigh 249.68 ÷ 159.60 = 1.564 times as much.
Frequently asked questions
What is the difference between molar mass and molecular weight?
They are numerically the same. Molar mass has units of g/mol; molecular weight (relative molecular mass) is a ratio with no units. Labs use them interchangeably.
Why does my bottle show a slightly different molecular weight?
Suppliers may use older or more precise atomic weights. Differences are usually in the second decimal place and do not matter for routine solution preparation.
How do I account for purity?
Divide the mass to weigh by the purity fraction. For example, 5.844 g of NaCl at 99.5% purity needs 5.844 ÷ 0.995 = 5.873 g.
Does it work for organic compounds?
Yes. Enter the molecular formula, e.g. C6H12O6 for glucose or C9H8O4 for aspirin. Structural formulas like CH3COOH also work.