Molar Mass Calculator
Enter a chemical formula; the tool instantly computes the molar mass M = Σ(atomic mass × atom count) from atomic weights, and also reports element composition and percentage by mass.
Input Data
Results
At a glance:The molar mass (symbol M) of a substance is the mass of one mole of it, numerically equal to its relative molecular mass (for ionic compounds, the relative formula mass), unit g/mol. The core of computing it is: 'sum each element's atomic weight times its atom count in the formula', i.e. M = Σ(atomic mass × count). Atomic weights come from the periodic table (the relative atomic masses of the elements, today on the unified atomic mass unit scale); the count is given by the subscripts in the chemical formula, and a group in brackets multiplies the whole group. Using this tool's default: water H₂O = 2×1.008 (H) + 15.999 (O) = 18.015 g/mol; table salt NaCl = 22.990 (Na) + 35.45 (Cl) = 58.44 g/mol; slaked lime Ca(OH)₂ = 40.078 + 2×(15.999 + 1.008) = 40.078 + 2×17.007 = 74.092 g/mol; blue vitriol CuSO₄·5H₂O = 63.546 (Cu) + 32.06 (S) + 4×15.999 (O) + 5×(2×1.008 + 15.999) (water of crystallisation) = 249.6 g/mol. The molar mass is the bridge from formula to grams — with it you can convert grams to moles (n = m/M), compute how much of each reactant by the reaction equation, and derive mass percentages. The 'mass percentage' of each element = (atomic mass × count) / M × 100%, describing composition; summing all to 100% is the basis of analysis. Note when computing: first, read the formula correctly — subscripts after an element are its count, brackets multiply the whole group, and a dot '·' indicates water of crystallisation (included in mass). Second, use the periodic-table atomic weights (with one decimal usually enough); polymers and hydrates need their full formula. Third, molar mass is an intensive property, independent of how much substance you take. Fourth, for mixtures use the average molar mass (mole-fraction weighted). In short, this calculator turns a chemical formula into molar mass and composition in one step — the foundation of all stoichiometry (moles, grams and reaction equations).
Formula
Molar mass: M = Σ (atomic mass × atom count) in the formula.
Numerically equals the relative molecular mass (or formula mass).
Mass percent: w% = (atomic mass × count) / M × 100%.
Hydrates: include the ·nH₂O waters of crystallisation in the mass.
$$M = \sum_i (\text{atomic mass}_i \times \text{count}_i)$$How to Use
- Enter the chemical formula, e.g. H₂O, CO₂, NaCl, Ca(OH)₂, CuSO₄·5H₂O.
- The right panel instantly shows the molar mass M (g/mol).
- The tool also reports element composition and mass percentages (%) of each element.
Molar masses of common substances
| Substance | Formula | Molar Mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Carbon dioxide | CO₂ | 44.01 |
| Sodium chloride | NaCl | 58.44 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Sulphuric acid | H₂SO₄ | 98.08 |
| Copper(II) sulphate pentahydrate | CuSO₄·5H₂O | 249.6 |
M = Σ(atomic mass × count); hydrates include the waters of crystallisation.
Case Studies
Water and salt
H₂O = 2×1.008 + 15.999 = 18.015 g/mol.
NaCl = 22.990 + 35.45 = 58.44 g/mol.
These are the most common molar masses, the basis of grams-to-moles conversion.
Hydrate with water of crystallisation
Copper(II) sulphate pentahydrate CuSO₄·5H₂O = 63.546 + 32.06 + 4×15.999 + 5×(2×1.008+15.999) = 249.6 g/mol.
The five waters of crystallisation account for 5×18 = 90 g/mol of mass — ignoring them would give the wrong molar mass.
So for hydrates, the '·nH₂O' must be included in the computation.
FAQ
How is molar mass computed?
Sum each element's atomic weight times its atom count in the formula: M = Σ(atomic mass × count). Bracket groups multiply the whole group; a dot '·' indicates water of crystallisation and is included. Atomic weights come from the periodic table.
Is molar mass the same as relative molecular mass?
Numerically yes. The relative molecular (or formula) mass is a ratio without units, while molar mass uses g/mol; the number is the same. E.g. water's relative molecular mass is 18.015, molar mass 18.015 g/mol.
Does a hydrate include the water of crystallisation?
Yes. A hydrate formula like CuSO₄·5H₂O includes 5 water molecules; their mass (5×18 = 90) must be summed, otherwise the molar mass is wrong — a common pitfall with salts and analytical reagents.
Does molar mass depend on how much I take?
No. Molar mass is an intensive property, independent of the amount taken; 1 mol of water is 18 g and 2 mol is 36 g, but the molar mass is always about 18.015 g/mol.
Why is molar mass important?
It is the bridge from chemical formula to grams: with M you convert grams to moles (n = m/M), compute reactant amounts from the equation and derive mass percentages. It is the foundation of every stoichiometric calculation.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.