Molar Mass Calculator from Chemical Formula

What molar mass is and how it is calculated

Molar mass is the mass of one mole of a substance, that is of 6.022·10²³ formula units, and is measured in grams per mole. Numerically it matches the molecular mass expressed in atomic mass units: water has a molecular mass of 18.015 u and a molar mass of 18.015 g/mol. That coincidence is what makes the mole useful, because it lets you move from the world of atoms to the world of the balance without changing the number.

The calculation is a weighted sum: multiply each element's atomic mass by how many times it appears in the formula and add the contributions. The delicate part is reading the formula correctly. In Ca(OH)₂ the subscript 2 applies to the whole OH group, so there are two oxygens and two hydrogens, not one oxygen and two hydrogens. In hydrates such as CuSO₄·5H₂O the coefficient 5 multiplies the entire water molecule that follows, adding five oxygens and ten hydrogens.

The percentage composition follows by dividing each element's contribution by the total molar mass. It is the figure you need to go from a formula to how much product you get, and in the other direction to work out an empirical formula from an elemental analysis.

Common mistakes

Ignoring brackets: Al₂(SO₄)₃ has twelve oxygens, not four. The subscript outside the bracket multiplies everything inside it.
Getting the capitalisation of a symbol wrong: CO and Co are different substances, as are NO and No. The second letter of a symbol is always lowercase.
Forgetting the water of crystallisation: anhydrous copper sulfate has a molar mass of 159.6 g/mol, the pentahydrate 249.7. Weighing out an amount based on the wrong formula ruins the whole preparation.

Frequently asked questions

How do you calculate the molar mass of a compound?

Multiply each element's atomic mass by the number of atoms in the formula and add the results. For H₂O: 2 × 1.008 + 15.999 = 18.015 g/mol.

What is the difference between molar mass and molecular mass?

The number is the same, the units and the meaning differ. Molecular mass is the mass of a single molecule in atomic mass units; molar mass is the mass of one mole of molecules in grams.

How do you write a hydrated salt in the calculator?

With a dot between the anhydrous part and the water, for example CuSO4·5H2O. An asterisk or a plain full stop works too. The number before H2O multiplies the whole water molecule.

Does the calculator know every element?

Yes, all 118 elements of the periodic table, with the IUPAC conventional atomic weights. For elements with no stable isotope it uses the mass number of the longest-lived one, as textbooks do.

How this calculation works

Molar mass: M = Σ (nᵢ · Aᵢ), where nᵢ is the number of atoms of element i in the formula and Aᵢ its relative atomic mass, in g/mol. Percentage composition of element i: %ᵢ = (nᵢ · Aᵢ)/M × 100. Subscripts inside a bracket are multiplied by the outer subscript; in hydrated salts the coefficient before the water molecule multiplies all of its atoms. The atomic weights are the IUPAC conventional values; for radioactive elements with no defined isotopic composition, the mass number of the most stable isotope is used.

Periodic table

All 118 elements, searchable by name, symbol or atomic number, each with its full record.

Moles

Conversion between grams, moles, particles and volume of gas.

Concentration and dilution

Molarity, molality, percentages, ppm and dilution M₁V₁ = M₂V₂.

pH and pOH

pH, pOH and the concentrations of H⁺ and OH⁻ ions.