scriptiii
100% Client-Side

Σ atomic weights

Molar mass calculator

A chemical formula gives the molar mass, each element’s contribution and the atom count — brackets and hydrate water included.

Examples: H2O · NaCl · Ca(OH)2 · Al2(SO4)3 · C6H12O6

Short answer: copper sulfate pentahydrate CuSO₄·5H₂O has a molar mass of249.68 g/mol, of which the five water molecules contribute 90.08 g/mol. That is why the blue crystals lose 36% of their mass when heated — the water of crystallisation boils off.

Where the number comes from

A molar mass is the sum of the atomic weights of every atom in the formula unit. It is the bridge between the balance and the reaction: 249.68 g of the hydrate is one mole, so 24.97 g is 0.1 mol, which is the amount the mole calculator and the molarity calculator then work with.

Worked example

Sodium chloride is 22.98977 + 35.45 = 58.44 g/mol. To make 250 mL of a 0.4 mol/L solution you need 0.1 mol, so 5.844 g weighed out and made up to the mark — the same arithmetic the molarity page performs in the opposite direction.

Molar mass questions

Which atomic weights are used?

The CIAAW Standard Atomic Weights 2024 — the same values printed on a periodic table. For the sixteen elements whose weight is published as a range (hydrogen, carbon, oxygen and others) the conventional single value is used, which is the normal convention for molar mass.

Why are some elements missing?

Technetium, promethium and the heaviest radioactive elements have no recommended standard atomic weight. The page says so and refuses the formula rather than printing a number that would be wrong for a real sample.

What does the calculator accept?

Nested brackets and hydrates: Ca(OH)2, Al2(SO4)3 and CuSO4·5H2O all parse. A coefficient at the front, as in 2H2O, is refused because a molar mass belongs to the formula, not to an amount of it.

How is the half-mass used?

For acids and bases it is the equivalent weight in the simple cases — the mass per mole of reactive units — so it is reported for titration work. For a polyprotic acid the equivalent depends on how many protons take part, which is a chemistry decision the number cannot make for you.

Related tools

Mole calculatorMass, moles and particles.Molarity calculatorMoles per litre of solution.Dilution calculatorC₁V₁ = C₂V₂.