Molar mass calculator
The molar mass of a compound is the sum of the standard atomic weights of its atoms, in g/mol: for water, H₂O = 2 × 1.0080 + 15.999 = 18.015 g/mol. Type a formula, including brackets, hydrates (CuSO4·5H2O or CuSO4*5H2O) and decimal counts, and get the molar mass with each element’s count, atomic weight and mass percentage, using the IUPAC (CIAAW) 2021 standard atomic weights.
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At a glance
| Formula | M = Σ nᵢ × Aᵣ(i) |
|---|---|
| Atomic weights | CIAAW standard atomic weights 2021 (84 elements); abridged values where the CIAAW gives an interval |
| Reads | Brackets ( ) [ ] { }, hydrates and adducts (·, *, .), decimal counts; charges ignored |
| Output | Molar mass, Hill formula, count, atomic weight, g/mol and mass % per element; CSV |
| Runs in | Your web browser; nothing is uploaded |
| Price | Free, no account needed |
What it does
How to use the molar mass calculator
- Type the formulaFor example C6H12O6, Ca(OH)2, K4[Fe(CN)6] or CuSO4·5H2O. Symbols are case-sensitive: Co is cobalt, CO is carbon monoxide.
- Check how it was readThe formula is shown back with subscripts, so a typo shows up at once.
- Use the resultRead the molar mass and the mass % of each element, work out the mass of an amount in moles, or copy the value into the molarity calculator.
How it works
M = Σ nᵢ × Aᵣ(i) mass %ᵢ = 100 × nᵢ × Aᵣ(i) / M
where nᵢ is the number of atoms of element i in the formula and Ar(i) its standard atomic weight, which in g/mol is the molar mass of the element. Groups in brackets are multiplied by the number after them, and the parts of a hydrate or adduct by the number in front (5H2O is five waters).
The atomic weights are the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem., 2022; table at ciaaw.org). For hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, argon, bromine, thallium and lead the CIAAW gives an interval, because their isotopic composition varies in nature; the calculator uses the CIAAW’s abridged value (for example C = 12.011, O = 15.999, Cl = 35.45) and shows the interval. Elements without a standard atomic weight (those with no stable isotopes, such as technetium) cannot be used. A charge written at the end of a formula (SO4^2−) is ignored.
Worked example
Copper(II) sulfate pentahydrate, CuSO₄·5H₂O. Cu 1 × 63.546 + S 1 × 32.06 + O 9 × 15.999 + H 10 × 1.0080 = 63.546 + 32.06 + 143.991 + 10.080 = 249.677 g/mol. Copper is 63.546 / 249.677 = 25.45 % of the mass, and the five waters 90.075 g/mol of it.
Glucose, C₆H₁₂O₆. 6 × 12.011 + 12 × 1.0080 + 6 × 15.999 = 180.156 g/mol, of which carbon is 40.00 %.
Frequently asked questions
How do you calculate molar mass?
Multiply the atomic weight of each element by the number of its atoms in the formula and add them up. For NaCl: 22.98976928 + 35.45 = 58.440 g/mol.
Is molar mass the same as molecular weight?
They have the same number. Molar mass is in g/mol; molecular weight (relative molecular mass) is the same quantity without units, and is often quoted in daltons for proteins.
How do I include water of crystallisation?
Write the hydrate with a centred dot, an asterisk or a full stop and the number of waters: CuSO4·5H2O, CuSO4*5H2O or CuSO4.5H2O. If you weigh the hydrate, use the hydrate’s molar mass.
Why does the calculator give a slightly different value from my bottle?
Suppliers may round differently or use older atomic weights. The calculator uses the CIAAW 2021 standard atomic weights; for elements whose weight varies in nature (such as H, C, O, S and Cl) it uses the CIAAW’s abridged value, so the last decimal place can differ.
Can I use non-whole numbers, such as in Fe0.95O or LiNi0.8Mn0.1Co0.1O2?
Yes. Counts can be decimals; a dot is read as a decimal point unless water follows it (CuSO4.5H2O) or a bracket follows it directly.
How do I find the mass percentage of an element in a compound?
Divide the element’s contribution (atoms × atomic weight) by the molar mass and multiply by 100. The table under the result lists it for every element.
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