Molarity calculator
Solve for mass to weigh, molar concentration, or final volume. Look up molecular weight by reagent name or formula — waters of hydration included — or key in the value from your CoA.
The formula
m = C × V × MW
- m
- mass of solute to weigh, in grams
- C
- molar concentration, in mol/L
- V
- final solution volume, in liters
- MW
- molecular weight of the solute as weighed, in g/mol
Worked example
Prepare 500 mL of 0.1 M copper(II) sulfate
Copper(II) sulfate is almost always on the shelf as the pentahydrate, CuSO4·5H2O, MW 249.68 g/mol — not the anhydrous salt at 159.60 g/mol. The MW that matters is the MW of what lands on the balance pan.
m = 0.1 mol/L × 0.500 L × 249.69 g/mol = 12.48 g of the pentahydrate, dissolved and made up to 500 mL in a volumetric flask.
Weighing 12.48 g of anhydrous CuSO4 instead would give a solution roughly 56% too concentrated — the classic hydrate mistake.
Common pitfalls
Using the anhydrous MW for a hydrated reagent
The waters of hydration are on the balance too. Always use the molecular weight of the form you are weighing, exactly as written on the bottle label.
Adding the volume instead of making up to volume
Molarity is defined against final solution volume. Dissolve in less solvent than the target, then fill to the mark in a volumetric flask — do not add the full volume of solvent to the solid.
Rounding the molecular weight too early
Carry full precision through the calculation and round only the final answer. This calculator does exactly that — the copy button gives you the unrounded value.