Molar Mass Calculator | Molecular Weight

The Molar Mass Calculator computes the molecular weight of any chemical compound by parsing the formula and summing the standard atomic masses of all atoms. Enter any formula, from simple H₂O to complex compounds like Al₂(SO₄)₃, and get the molar mass in g/mol with a complete element-by-element breakdown and percent composition by mass.

• ›70+ elements: covers the full periodic table through uranium, using IUPAC 2021 standard atomic weights from ciaaw.org.
• ›Bracket parsing: handles nested brackets, Ca(OH)₂, (NH₄)₂SO₄, Al₂(SO₄)₃, with unlimited nesting depth.
• ›Percent composition: each element row shows its mass contribution and percentage of total molar mass as a visual bar.
• ›Quick presets: one-click access to 12 common compounds including water, glucose, aspirin, and sulfuric acid.

What Is Molar Mass and Why Does It Matter?

Molar mass is the mass of one mole (6.02214076 × 10²³ particles, Avogadro's number) of a substance, expressed in grams per mole (g/mol). It numerically equals the average atomic or molecular weight measured in atomic mass units (u or amu). Molar mass is the essential bridge between the macroscopic world of grams on a balance and the microscopic world of atoms and molecules.

In chemistry, almost every quantitative calculation, stoichiometry, solution preparation, yield calculations, begins with converting a mass measurement into moles using: moles = mass (g) ÷ molar mass (g/mol). Knowing the molar mass precisely is therefore fundamental to accurate experimental results.

• ›Stoichiometry: use molar mass to convert between grams and moles in reaction equations.
• ›Solution preparation: weigh out exactly the right mass of solute to achieve a target molarity.
• ›Empirical vs molecular formula: use combustion analysis data and molar mass to find the molecular formula.
• ›Gas law calculations: molar mass connects mass measurements to moles for PV = nRT applications.

Standard Atomic Weights, IUPAC 2021

The atomic masses used in this calculator are the 2021 IUPAC standard atomic weights, published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW, ciaaw.org). These values reflect the natural isotopic composition of each element in terrestrial samples and are revised periodically as measurement precision improves.

• ›Standard atomic weight reflects natural isotopic abundance, not the mass of a single isotope.
• ›Hydrogen: 1.008 (range 1.00784–1.00811 in natural samples); conventional value used.
• ›For radioactive elements with no stable isotopes, the mass of the most stable known isotope is used.
• ›These values match NIST, CRC Handbook, and standard chemistry textbook data.

Percent Composition by Mass

Percent composition tells you what fraction of the compound's total molar mass comes from each element: % element = (mass contribution ÷ total molar mass) × 100. For water (M = 18.015 g/mol): %H = 2.016/18.015 × 100 = 11.19%; %O = 15.999/18.015 × 100 = 88.81%.

Percent composition is used to determine empirical formulas from combustion analysis data, to verify compound purity by elemental analysis, and in materials science to confirm alloy and ceramic compositions.

Molar Mass in Lab Practice

When preparing a standard solution, you need to weigh out the right number of moles of solute. The workflow is: target molarity × target volume (L) = moles needed; moles × molar mass = grams to weigh. For example, 500 mL of 0.1 M NaCl: 0.1 × 0.5 = 0.05 mol; 0.05 × 58.443 = 2.922 g of NaCl.

• ›Use molar mass to 3–4 decimal places for accurate solution preparation.
• ›Hydrated salts have a larger molar mass than anhydrous salts, CuSO₄·5H₂O ≠ CuSO₄.
• ›For gases: moles = mass ÷ molar mass; volume at STP = moles × 22.414 L/mol.
• ›Mass spectrometry measures molar mass experimentally via the molecular ion (M⁺) peak.