Molar Mass Calculator

Written byOleh Soloninupdated 5 October 2026

Chemical formula

Brackets and hydrates with an asterisk: Ca(OH)2, CuSO4*5H2O.

Sample mass, g

Needed for the amount of substance.

Result

Molar mass, g/mol249.677
Mass fraction of each element
Cu · 1 atom · 63.546 g/mol · 25.45% S · 1 atom · 32.06 g/mol · 12.84% O · 9 atoms · 143.991 g/mol · 57.67% H · 10 atoms · 10.08 g/mol · 4.04%
Working63.546 + 32.06 + 9 × 15.999 + 10 × 1.008 = 249.677
Amount of substance, molenter a mass
Atoms in the formula21
FormulaCuSO4*5H2O
Mass of one mole, g249.677

The sum of the atomic masses, nothing more

Works out molar mass from a chemical formula, handles brackets and hydrates, and gives the mass fraction of every element.

Molar mass is the sum of the atomic masses of every atom in a formula, expressed in grams per mole. It matches the relative molecular mass numerically, so water is 18.015 both in atomic mass units and in grams per mole.

The only difficulty is parsing. Brackets multiply everything inside them, and the dot before a hydrate multiplies the part that follows. CuSO4·5H2O is not one molecule but copper sulfate with five water molecules in its crystal lattice.

How to use

Enter the formula, with brackets, subscripts and hydrates written after an asterisk.
Add a sample mass if you want the amount in moles.
Read the molar mass and the composition by element in percent.

Hydrates and the weight of their water

Copper sulfate pentahydrate, CuSO4*5H2O, has a molar mass of 249.68 g/mol, of which the five waters account for 90.08 g — more than a third. Weighing out the anhydrous salt instead of the hydrate for the same amount of substance therefore needs a very different number on the balance.

The elemental breakdown comes from the same formula: oxygen carries 57.67% of the mass and copper 25.45%. Those percentages are what an analysis is compared against.

CuSO4*5H2O, molar mass 249.677 g/mol. Worked out with this calculator.
Element	Atoms	Mass, g/mol	Share
Cu	1	63.546	25.45%
S	1	32.06	12.84%
O	9	143.991	57.67%
H	10	10.08	4.04%

What this calculator shows

Molar mass to four decimal places
The mass fraction of each element as a percentage
The amount of substance, if a sample mass is given

What to keep in mind

Standard IUPAC atomic weights are used — averages over the natural isotopic composition.
Round and square brackets are parsed, as are hydrates written with a dot or an asterisk.

Where the numbers come from

Checked against IUPAC standard atomic weights.

FAQs

How do I calculate molar mass?

Add up the atomic masses of every atom in the formula. For H2SO4 that is 2 × 1.008 + 32.06 + 4 × 15.999 = 98.07 g/mol.

How does molar mass differ from molecular mass?

They match numerically; the units differ. Relative molecular mass is dimensionless, the mass of a molecule is expressed in daltons, and molar mass in grams per mole.

How do I write a hydrate?

With an asterisk or a middle dot: CuSO4*5H2O or CuSO4·5H2O. Everything after it is multiplied by the coefficient and added to the total.

Why are atomic masses fractional?

Because they are weighted averages over natural isotopes. Chlorine is 35.45 because isotopes 35 and 37 occur in roughly a three to one mix.

What is the molar mass of copper sulfate?

249.68 g/mol for the pentahydrate CuSO4*5H2O. The anhydrous salt is 159.6 g/mol; the 90.08 g difference is the water.

What is the percentage composition for?

Checking an analysis against theory, and working out how much of an element a given mass contains — which is exactly what fertiliser and reagent labels are quoting.

Worked example

Copper sulfate pentahydrate

Input: CuSO4*5H2O

Output: 249.677 g/mol across 21 atoms

Note: There are nine oxygen atoms in the formula: four in the sulfate and five in the water of crystallisation. They account for 57.67 percent of the mass — more than copper and sulfur combined.

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Result

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