Molar Mass Calculator

Calculate molar mass of chemical formulas with element breakdown

What is it and how does it work?

The molar mass of a compound is the mass (in grams) of exactly one mole (6.022 × 10²³ particles) of that substance. It's the bridge between the macroscopic world of grams you can weigh on a scale and the microscopic world of atoms and molecules. For H₂O: hydrogen has molar mass 1.008 g/mol, oxygen has 16.00 g/mol, giving water a molar mass of 18.016 g/mol. This means 18.016 grams of water contains exactly 6.022 × 10²³ water molecules.

This calculator parses a chemical formula (like C₆H₁₂O₆ for glucose or NaCl for salt) and sums the atomic masses of all constituent atoms using the IUPAC standard atomic weights. The result is essential for stoichiometry: calculating how many grams of a reactant you need, converting between grams and moles, or finding the percent composition of elements in a compound.

Common use cases

• Stoichiometry preparation: prepare a 0.5 M solution of glucose (C₆H₁₂O₆, MW = 180.16 g/mol) — dissolve 90.08 g in enough water to make 1 litre.
• Percent composition: find the mass percentage of nitrogen in urea (CH₄N₂O, MW = 60.06 g/mol) — nitrogen contributes 2×14.007 = 28.014 g/mol, so N% = 28.014/60.06 × 100 = 46.6%.
• Empirical to molecular formula: if combustion analysis gives 40% C, 6.7% H, 53.3% O with MW ≈ 180, the molecular formula is C₆H₁₂O₆ (check: 6×12 + 12×1 + 6×16 = 180).

Frequently asked questions

What is the difference between molar mass and molecular weight?

They are numerically the same but conceptually different. Molecular weight is dimensionless (the ratio of molecular mass to 1/12 the mass of carbon-12). Molar mass has units of g/mol. In practice, chemists use them interchangeably: "the molecular weight of water is 18.015 g/mol".

Why do atomic masses have decimal values?

Elements exist as natural mixtures of isotopes with different masses. Carbon is 98.9% ¹²C (mass 12.000) and 1.1% ¹³C (mass 13.003). The standard atomic weight (12.011) is the abundance-weighted average. This is why you see decimals in periodic table values.

How do I handle hydrates in the molar mass calculation?

Hydrates are written as CuSO₄·5H₂O (copper(II) sulfate pentahydrate). Calculate the molar mass of CuSO₄ (159.61) plus 5 × 18.015 (for 5 water molecules) = 249.68 g/mol. The dot notation means the water molecules are loosely bound in the crystal structure.

What is the mole and why 6.022 × 10²³?

One mole is defined as exactly 6.02214076 × 10²³ elementary entities (Avogadro's number, fixed by the 2019 SI redefinition). It was chosen so that 1 mole of ¹²C weighs exactly 12 grams — making molar mass numerically equal to atomic/molecular weight in atomic mass units (u or Da).

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