⚛️ Atomic Mass Calculator – Molar Mass, Isotopes & Percent Composition
Whether you are balancing a stoichiometry problem, preparing a lab reagent, or interpreting a mass spectrometry spectrum, accurate atomic and molecular masses are the foundation of every calculation. This tool brings four closely related calculations into one place: molar mass from a chemical formula, average atomic mass from isotope data, monoisotopic mass, and percent elemental composition.
Molar Mass from a Chemical Formula
The molar mass of a compound is the sum of the standard atomic weights of all atoms in one formula unit, expressed in g/mol. Enter any formula — simple (NaCl), multi-group (Ca(OH)₂), nested (Al₂(SO₄)₃), or hydrated (CuSO₄·5H₂O) — and the parser validates every element symbol against the full 118-element periodic table before computing the result.
Example for H₂SO₄:
H: 2 × 1.008 = 2.016 g/mol
S: 1 × 32.06 = 32.060 g/mol
O: 4 × 15.999 = 63.996 g/mol
─────────────────────────────
Total = 98.072 g/molAverage Atomic Mass from Isotope Data
Each natural element is a mixture of isotopes. The average atomic mass is the weighted mean across all stable isotopes:
A_avg = Σ (mᵢ × aᵢ)where mᵢ is the exact isotope mass in amu and aᵢ is its fractional abundance (abundance% ÷ 100). All abundances must sum to 100% (±0.01%).
Example — Chlorine:
³⁵Cl: 34.96885 × 0.7577 = 26.496 amu
³⁷Cl: 36.96590 × 0.2423 = 8.958 amu
─────────────────────────────────────
Average atomic mass = 35.453 amuThis is exactly how IUPAC derives the standard atomic weights listed on the periodic table. Use this mode to verify textbook values or to explore how abundance ratios affect the result.
Monoisotopic Mass
Monoisotopic mass uses only the most abundant stable isotope of each element instead of the weighted average. This matters in mass spectrometry, where high-resolution instruments record distinct isotope peaks rather than a blurred average. The difference between monoisotopic and average mass grows with molecular size — negligible for small molecules but several daltons for proteins.
Example — Ethanol (C₂H₅OH):
Average mass = 46.068 Da
Monoisotopic mass = 46.042 Da (using ¹²C, ¹H, ¹⁶O)
Difference = 0.026 DaPercent Composition by Mass
The percent composition shows each element's mass fraction in a compound:
% element i = (nᵢ × Aᵢ / M) × 100Example — Glucose (C₆H₁₂O₆, M = 180.16 g/mol):
| Element | Atoms | Mass (g/mol) | Composition |
|---|---|---|---|
| C | 6 | 72.066 | 40.00% |
| H | 12 | 12.096 | 6.71% |
| O | 6 | 95.994 | 53.29% |
Element Lookup
Enter any element symbol (e.g., Fe, Au, Cl) to instantly retrieve its atomic number, standard atomic weight (IUPAC 2021), and monoisotopic mass. Useful as a quick periodic-table reference without switching tabs.
Formula Syntax Reference
| Pattern | Example | Meaning |
|---|---|---|
| Element + subscript | H2O | 2 H atoms, 1 O atom |
| Parentheses group | Ca(OH)2 | 1 Ca, 2 O, 2 H |
| Square brackets | K4[Fe(CN)6] | Coordination complex |
| Hydrate (· or .) | CuSO4·5H2O | 5 water molecules added |
Fe not FE or fe). The calculator gives an immediate error for any unrecognised symbol.Unit Conversions
Results are shown in g/mol by default. You can also view the equivalent mass in:
- amu / Da — numerically identical to g/mol (1 g/mol ≡ 1 u per molecule)
- kg/mol — divide by 1 000
- Mass per single molecule (g) — divide by Avogadro's number (6.022 × 10²³)
Applications
- Stoichiometry — Convert between moles and grams in reaction calculations
- Reagent preparation — Weigh out the correct amount of a solid compound to make a target molar solution
- Mass spectrometry — Predict monoisotopic peaks for compound identification
- Isotope geochemistry — Understand how isotopic mixing affects measured atomic weights
- Pharmaceuticals — Calculate molecular weight of drug candidates for formulation and dosage calculations