Molar Mass Calculator — Compound Names, Hydrates, Breakdown

Molar Mass Calculator

Formulas · hydrates · compound names · NIST atomic weights

Chemical Formula

Try: H₂O H₂SO₄ Glucose CuSO₄·5H₂O NaHCO₃ baking soda Al₂(SO₄)₃

Molar Mass

Composition Breakdown

Quantity Conversions

Mass g → — mol · — particles

Moles mol → — g · — particles

Particles # → — mol · — g

Volume @ STP L → — mol · — g (gases only)

Solution g in L → — M (mol/L)

Need more than molar mass?

ElementVault includes 11 more chemistry calculators, an interactive periodic table, spectroscopy tools, reaction explorer, and a spaced-repetition study mode. Same compound parser, all in one app.

Open full workbench →

About molar mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). For a chemical compound, it equals the sum of the standard atomic weights of every atom in the formula, each multiplied by how many times that atom appears. The molar mass of water (H₂O), for example, is 2 × 1.008 + 1 × 15.999 = 18.015 g/mol.

The atomic weights this calculator uses come from the NIST Atomic Weights and Isotopic Compositions database — the same source IUPAC publishes and that every chemistry textbook ultimately traces back to. For most elements, the value used is the standard atomic weight averaged over naturally-occurring isotopes; for synthetic elements without a stable natural-isotope distribution, the mass of the longest-lived isotope is used instead.

Working with this calculator

The input accepts three kinds of entries:

Chemical formulas — type the formula directly: H2O, H2SO4, C6H12O6, Al2(SO4)3. Parentheses are nested correctly, so Ca(OH)2 resolves to Ca₁H₂O₂ rather than the common mis-parse Ca₁O₁H₂.
Hydrate notation — use a middle dot or period to separate the water of crystallization: CuSO4·5H2O, MgSO4·7H2O, AlK(SO4)2·12H2O. The coefficient before the water (5, 7, 12) is honored, and the result shows the anhydrous mass separately so you can weigh-out hydrated vs. anhydrous reagents without re-deriving.
Compound names — type the everyday name and the calculator looks it up: baking soda, epsom salt, aspirin, magnesium chloride hexahydrate. The built-in dictionary covers about 130 of the compounds students encounter in K-12 and intro-college chemistry, including common household names, lab reagents, and IUPAC-style "copper(II) sulfate pentahydrate" constructions.

The breakdown panel shows each element's contribution to the total mass and its percentage of the compound by weight — useful for elemental analysis, gravimetric calculations, and double-checking that your formula matches the compound you intended.

Avoiding common errors

Case matters. Chemical symbols are case-sensitive: Co is cobalt, but CO is carbon monoxide, and cO isn't anything. The calculator follows the standard convention (every element symbol starts with an uppercase letter, optionally followed by a single lowercase letter), so typos like NACL won't resolve.

Water of crystallization isn't optional. If you have a hydrated salt and you ignore the ·NH₂O portion, the molar mass you compute will be wrong — sometimes by 30% or more. Always use the formula that matches your actual reagent bottle.

The calculator is not a primary reference. For graduate research, regulatory documents, or any work where a fourth decimal place matters, look up the standard atomic weight at NIST directly. The values used here are accurate to within published IUPAC uncertainty, but precision-critical work should cite the primary source.

Frequently asked questions

What's the difference between molar mass and molecular weight?

Practically nothing. "Molecular weight" is the older, technically dimensionless term referring to the relative mass of a molecule compared to ¹²C; "molar mass" expresses the same number with units of g/mol. Most chemistry textbooks treat them as interchangeable for everyday calculations. Use whichever your course uses.

How do I find the moles in a given mass?

Divide the mass (in grams) by the molar mass (in g/mol). For example, 49 g of H₂SO₄ ÷ 98.072 g/mol = 0.4996 mol. The Quantity Conversions panel below the result does this automatically — type any mass, moles, or particle count and the other values fill in.

How does the calculator handle hydrates?

A middle dot (·) or period is treated as a multiplication separator. Each piece after the dot can have a leading coefficient indicating how many of those units are bound. So CuSO4·5H2O is parsed as one CuSO₄ plus five H₂O — total formula Cu₁S₁O₉H₁₀, total mass 249.677 g/mol.

Can I type compound names instead of formulas?

Yes — the built-in dictionary recognizes about 130 common compounds by name, including household terms (baking soda, epsom salt, bleach), pharmaceutical names (aspirin), and full IUPAC-style names (copper(II) sulfate pentahydrate). If a name doesn't resolve, type the formula instead.

Why do my textbook answers differ slightly?

Most discrepancies come from rounding — different textbooks use different precision for atomic weights (some use 1.01 for hydrogen, this calculator uses NIST's 1.008). For a typical small molecule the answers agree to the second decimal; for larger molecules cumulative rounding can shift the third decimal. Both answers are correct within their stated precision.

What unit is molar mass measured in?

Grams per mole (g/mol). This is the same numerical value as the molecule's mass in unified atomic mass units (u or Da), which is why the same number serves both for mass-per-molecule and mass-per-mole conversions.

Where do these atomic-weight values come from?

Every value used by this calculator originates from the NIST Atomic Weights and Isotopic Compositions database, which mirrors the IUPAC Commission on Isotopic Abundances and Atomic Weights' periodic review. Values are updated as new isotopic measurements refine the published standard atomic weights.