Molarity calculator
Give a formula, a mass and a volume and this works out the molarity, showing the molar mass it used. The one thing worth knowing before you start is which volume the formula means. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Up to a litre, not with a litre
Molarity is moles of solute per litre of solution. Not per litre of solvent. The two are different and the difference is not small.
Weigh out 58.44 g of sodium chloride, put it in a flask, and add water until the total volume reaches one litre. That is 1 mol/L. Weigh out the same salt and add one litre of water to it and you have something else — roughly 1.02 litres of solution, so about 0.98 mol/L.
Nothing in the arithmetic can catch this. Both ways of working give a perfectly sensible number. The formula has no way of knowing which volume you handed it, which is why the distinction has to be made in the glassware and not in the calculator.
Molarity and molality are different quantities
| Molarity (M) | moles of solute per litre of solution. Changes with temperature, because the solution expands. |
|---|---|
| Molality (m) | moles of solute per kilogram of solvent. Does not change with temperature, because mass does not. |
They are close for dilute aqueous solutions and diverge as things get concentrated. The abbreviations differ by a capital letter, which is the second time on this site that a capital letter has been the whole difference between two quantities.
Where these numbers come from
The atomic weights on this page are the abridged standard atomic weights 2024, © IUPAC / CIAAW — the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry, published at ciaaw.org and reproduced under their terms. CIAAW has published the critical evaluation of atomic weights since 1902.
The abridged table is the one used here rather than the full one, and that is a decision worth stating. Fourteen elements have no single atomic weight in the full table at all — they get an interval instead — and taking the middle of an interval would be this site inventing a number nobody published. CIAAW publish the abridged table with a single value and an uncertainty for exactly this purpose, and that is what you see.