Molar mass is the mass of one mole of a substance in grams per mole (g/mol). It equals the sum of atomic masses of all atoms in the chemical formula. One mole contains 6.022 times 10^23 particles (Avogadro's number).

What is the molar mass of water (H2O)?

H2O has molar mass 18.015 g/mol. Hydrogen (H) has atomic mass 1.008, and oxygen (O) has 15.999. Two H atoms plus one O atom: 2 times 1.008 + 15.999 = 18.015 g/mol.

What is the difference between molar mass and molecular weight?

Molar mass is expressed in g/mol and refers to one mole of a substance. Molecular weight is dimensionless (mass relative to carbon-12). Numerically they are the same — H2O molar mass is 18.015 g/mol and molecular weight is 18.015.

How do I enter a formula with parentheses?

Enter the formula as written: Ca(OH)2, Fe2(SO4)3. The calculator parses parentheses and subscripts recursively. Ca(OH)2 means one Ca plus two OH groups, giving Ca + 2O + 2H.

Molar Mass Calculator — Molecular Weight

Element	Count	Atomic mass	Contribution
H	2	1.008	2.0160
O	1	15.999	15.9990
Total molar mass	18.0150 g/mol

How this tool fits your workflow

Molar mass and why it matters

Molar mass bridges the microscopic world of atoms and the macroscopic world of laboratory measurements. You cannot count individual molecules, but you can weigh grams of a substance. Knowing the molar mass lets you convert between grams and moles, which is the foundation of stoichiometry — calculating how much reactant you need and how much product you will get.

For example: the reaction 2H2 + O2 → 2H2O. To produce 36 grams of water (2 moles, since molar mass of H2O = 18 g/mol), you need 4 grams of H2 (2 moles times 2 g/mol) and 32 grams of O2 (1 mole times 32 g/mol).

Common molar masses

Water (H2O): 18.015 g/mol. Carbon dioxide (CO2): 44.010 g/mol. Glucose (C6H12O6): 180.156 g/mol. Sodium chloride (NaCl): 58.443 g/mol. Sulfuric acid (H2SO4): 98.079 g/mol. Ammonia (NH3): 17.031 g/mol. Calcium carbonate (CaCO3): 100.086 g/mol.

The calculator handles any formula including complex parenthetical groups like Fe2(SO4)3 (iron(III) sulfate, 399.88 g/mol) and Ca3(PO4)2 (calcium phosphate, 310.18 g/mol). Just type the formula exactly as written in a chemistry textbook.

Frequently asked questions

What is molar mass?: Molar mass is the mass of one mole of a substance in grams per mole (g/mol). It equals the sum of atomic masses of all atoms in the chemical formula. One mole contains 6.022 times 10^23 particles (Avogadro's number).
What is the molar mass of water (H2O)?: H2O has molar mass 18.015 g/mol. Hydrogen (H) has atomic mass 1.008, and oxygen (O) has 15.999. Two H atoms plus one O atom: 2 times 1.008 + 15.999 = 18.015 g/mol.
What is the difference between molar mass and molecular weight?: Molar mass is expressed in g/mol and refers to one mole of a substance. Molecular weight is dimensionless (mass relative to carbon-12). Numerically they are the same — H2O molar mass is 18.015 g/mol and molecular weight is 18.015.
How do I enter a formula with parentheses?: Enter the formula as written: Ca(OH)2, Fe2(SO4)3. The calculator parses parentheses and subscripts recursively. Ca(OH)2 means one Ca plus two OH groups, giving Ca + 2O + 2H.