Molarity & Solution Prep Calculator
From the molar mass on the label, find the mass to weigh for a target molarity and volume — or the molarity, or the volume — with purity allowed for.
The grams to weigh out (corrected for purity), the moles, the molarity in M and mM, and g/L, with n = m/M and c = n/V shown so the prep can be checked.
Example: For 250 mL of 0.5 M with a molar mass of 58.44 g/mol (a sample value, the NaCl label figure): 0.125 mol → 7.305 g; at 95 % purity weigh 7.689 g. 10 g in 500 mL is 0.342 M.
Grams to moles
to moles per litre.
Two relations, where the molar mass comes from, and the bench details the page cannot know.
The arithmetic
Moles are mass divided by molar mass, n = m/M, and molarity is moles per litre of solution, c = n/V. Give any two of mass, molarity and volume and the page solves for the third; mass can be in g or mg and volume in mL or L, and results are also shown in mM and g/L. Molar mass is an input on purpose: the figure on the reagent bottle already accounts for the hydrate (CuSO₄·5H₂O is 249.69, not 159.61) and is the one you should use.
Purity and making up to volume
If the solid is not 100 % pure, the mass to weigh is the compound mass divided by the purity fraction; enter the assay percentage and the page reports both figures. Molarity refers to the final volume of solution, not the volume of solvent added: dissolve in less than the target volume, then make up to the mark in a volumetric flask.
What it does not cover
Molality (mol/kg solvent), normality, percent solutions and density-based conversions are different quantities and are not computed here; nor is anything about safety, dosing or medicine. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- IUPAC Gold Book — amount of substance, molar mass and amount concentration
- NIST SP 811 — the mole and SI units
Last reviewed 19 September 2026. How results are checked: How we verify.