Chemistry and laboratory
Molar mass calculator
Molar mass of any chemical formula, with brackets and hydrates such as CuSO4·5H2O, and the percent composition of each element.
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Result
- Molar mass
- 249.68 g/mol
- Formula
- CuSO₄·5H₂O
- Hill formula
- CuH₁₀O₉S
- Atoms per formula unit
- 21
Working
- M = Σ nᵢ × Ar(i) = 1 × 63.546 + 10 × 1.0080 + 9 × 15.999 + 1 × 32.06 = 249.68 g/mol
| Element | Atoms | Atomic weight (g/mol) | Mass (g/mol) | Mass percent |
|---|---|---|---|---|
| copper (Cu) | 1 | 63.546 | 63.55 | 25.45 % |
| hydrogen (H) | 10 | 1.0080 | 10.08 | 4.037 % |
| oxygen (O) | 9 | 15.999 | 143.99 | 57.67 % |
| sulfur (S) | 1 | 32.06 | 32.06 | 12.84 % |
- Rounded to 2 decimal places, the precision of the least precise abridged atomic weight used (S: 32.06). Unrounded sum: 249.677 g/mol.
Method and limits
Type a formula and the molar mass appears as you type. Brackets ( ), [ ] and { } can be nested and multiplied, and the parts of a hydrate or adduct are joined with a middle dot, an asterisk or a full stop, each with its own multiplier (CuSO4·5H2O, CaSO4·0.5H2O). A charge written at the end (SO4^2-, NH4+, Fe³⁺) and a state such as (aq) are ignored.
Method
M = Σ nᵢ × Ar(i) (g/mol)
Each element’s count nᵢ is multiplied by its standard atomic weight Ar and the products are added. The percent composition of an element is its share of that sum: wᵢ = nᵢ Ar(i) ÷ M × 100 %.
Atomic weights are the CIAAW 2024 abridged standard atomic weights, given to five significant figures. For elements whose atomic weight varies in nature (hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, bromine, thallium) the abridged table gives a conventional value, such as 1.0080 for H and 12.011 for C. The result is rounded to the decimal places of the least precise atomic weight used, as addition requires: sulfur (32.06) and chlorine (35.45) limit a result to two decimals.
Limits
- Elements with no stable isotopes (technetium, promethium, polonium and the elements after bismuth except thorium, protactinium and uranium) have no standard atomic weight, so the tool asks for a known molar mass instead.
- Isotope-labelled compounds (D2O, ¹³C) need isotope masses, which this tool does not use.
- Decimal subscripts (non-stoichiometric Fe0.95O) are not accepted; a decimal multiplier is allowed after a hydrate dot.
Worked example
CuSO4·5H2O: 63.546 + 32.06 + 9 × 15.999 + 10 × 1.0080 = 249.677, rounded to 249.68 g/mol because sulfur’s weight has two decimals. Water is 18.015 g/mol: 11.19 % hydrogen and 88.81 % oxygen by mass.
References
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