Molar mass calculator

Numerically, yes. Molar mass carries units of g/mol, while molecular weight (relative molecular mass, Mr) is the same number without units, measured against one twelfth of the mass of a carbon-12 atom. Water has a molar mass of 18.015 g/mol and a relative molecular mass of 18.015. For ionic solids such as NaCl, which have no discrete molecules, the usual term is formula mass.

Multiply each element's atom count by its atomic weight and add the products. For water, H2O: 2 × 1.008 + 15.999 = 18.015 g/mol. Brackets multiply everything inside them, so Ca3(PO4)2 holds 2 phosphorus and 8 oxygen atoms. A hydrate's water counts in full: CuSO4·5H2O adds 10 hydrogen and 5 oxygen atoms to copper sulfate, for 249.677 g/mol.

Atomic weights are revised as isotope measurements improve, and books round them differently. IUPAC's 2021 abridged table gives sulfur as 32.06 where older tables used 32.065, so copper sulfate pentahydrate comes to 249.677 g/mol here and 249.69 g/mol on PubChem. A difference in the second decimal place is normal; use whichever table your course or method specifies.

It is the share of a compound's mass that each element contributes: the element's mass in one formula unit divided by the molar mass, times 100. Glucose is 40.00 % carbon, 6.71 % hydrogen and 53.28 % oxygen. Chemists compare these figures with percentages measured by combustion analysis to confirm a sample's identity or work out an empirical formula.

Every element symbol starts with a capital letter and any second letter is lowercase, so case decides which atoms you mean. Co is cobalt, 58.933 g/mol, while CO is carbon monoxide, one carbon and one oxygen atom, 28.010 g/mol. Likewise NO is nitric oxide but No is nobelium. Type formulas with standard capitalisation to get the compound you intend.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “Water” is checked against Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015).

IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values; PubChem compound pages (molecular weight cross-check).