Chemistry
Mass of one mole of a compound from its chemical formula — with the elemental composition alongside.
Element symbols with counts; parentheses allowed, e.g. Ca(OH)2. Results update as you type. All calculations run locally in your browser.
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Molar mass = Σ (count of each element × that element's atomic weight). Atomic weights are the standard relative atomic masses: H 1.008, C 12.011, N 14.007, O 15.999, and so on. Parentheses multiply everything inside them, so Ca(OH)2 means one calcium plus two of everything in (OH): Ca + 2 × O + 2 × H. The result is in grams per mole (g/mol).
Water, H2O: 2 × 1.008 (hydrogen) + 1 × 15.999 (oxygen) = 2.016 + 15.999 = 18.015 g/mol. So one mole of water — 6.022 × 10²³ molecules — weighs 18.015 grams, about a tablespoon. By composition, water is 2.016 ÷ 18.015 = 11.19% hydrogen and 15.999 ÷ 18.015 = 88.81% oxygen by mass. Other checks: CO2 = 12.011 + 2 × 15.999 = 44.009 g/mol; table salt NaCl = 22.990 + 35.45 = 58.440 g/mol.
This page covers common elements with standard atomic weights; it does not handle charges (ions), isotopes (deuterated water needs D as a separate entry, which is not included), hydrates written with a dot (write CuSO4·5H2O as CuSO4H10O5 instead), or structural formulas. Atomic weights are averages over natural isotope abundances, so they vary slightly by source — the third decimal may differ between tables. For laboratory work, confirm against your reference table; for teaching and estimation, these values are standard.
A counting unit: one mole is 6.022 × 10²³ particles (Avogadro's number). Molar mass connects that count to something weighable — the grams in one mole of a substance.
Because atomic weights are not whole numbers: hydrogen averages 1.008 (mostly H-1 with a little heavier H-2) and oxygen 15.999. Whole-number masses are a school shortcut; 18.015 is the standard value.
The number after a closing parenthesis multiplies everything inside it. In Ca(OH)2, the 2 doubles both O and H: Ca + O2 + H2, giving 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol.
The page lists the elements it knows and reports unknown symbols instead of guessing. The table covers the elements most formulas use; rare earths and synthetics are left out deliberately rather than given approximate weights.
To convert mass to moles, divide grams by molar mass; to convert moles to mass, multiply. For example, 36.03 g of water is 36.03 ÷ 18.015 = 2.00 moles.
Numerically yes, units differ: molecular weight is a relative number (unitless), molar mass is that number expressed in g/mol. People use the terms interchangeably in most contexts.
Results are estimates only and do not constitute professional advice.