Molecular weight calculator
Molar mass from a formula or reagent name, with the elemental composition and — for a hydrate — the anhydrous comparison that decides what you actually weigh.
The formula
MW = Σ nᵢ Aᵢ mass % = 100 × nᵢAᵢ / MW
- MW
- molecular weight, g/mol
- nᵢ
- number of atoms of element i in the formula
- Aᵢ
- conventional atomic weight of element i, g/mol (IUPAC 2021)
Worked example
Copper(II) sulfate pentahydrate, CuSO₄·5H₂O
The anhydrous salt is Cu 63.546 + S 32.06 + 4 × O 15.999 = 159.60 g/mol. Five waters add 5 × 18.015 = 90.075, giving 249.68 g/mol for the pentahydrate.
Water is therefore 90.075 / 249.68 = 36.08% of the mass, and the correction factor is 249.68 / 159.60 = 1.5644. A method calling for 1.000 g of anhydrous CuSO₄ needs 1.564 g of the pentahydrate weighed out.
Get that backwards and the standard is 36% weaker than intended, so every result quantified against it reads 56% high — which no acceptance criterion will absorb.
Common pitfalls
Case matters: Co is cobalt, CO is carbon monoxide
A case-insensitive formula parser returns a perfectly plausible molecular weight for entirely the wrong compound. This one is case-sensitive and refuses what it cannot read, which is the safer failure.
Weighing a hydrate against the anhydrous molecular weight
Reagent bottles are frequently hydrates while methods are frequently written for the anhydrous salt. Check the label against the formula in the method every time; the comparison panel above gives you the factor.
Mass percentages need not sum to exactly 100
Rounded to four significant figures the column may total 99.99 or 100.01. That is rounding, not error. We do not adjust the last digit to make the column look tidy — full precision is preserved internally and available on copy.
Conventional atomic weights are a convention, not a measurement
IUPAC publishes interval values for several elements because their isotopic composition varies by source. The tabulated weight is a convention agreed for general use, not a property of the material in your bottle. Where it matters, your CoA wins.
Molecular weight is not exact mass
Mass spectrometry works from monoisotopic (exact) mass, computed from the mass of the most abundant isotope of each element. That is a different quantity from the isotope-averaged molecular weight used for weighing, and this calculator does not compute it.