Molarity and molality calculator

Two concentration measures

Molarity is the amount of dissolved substance per litre of solution. Molality is the amount per kilogram of solvent. Since the denominators differ, their numerical values generally differ too. For example, 0.1 mol in a final solution volume of 0.5 L gives 0.2 mol/L. The same amount dissolved in 0.25 kg of solvent gives 0.4 mol/kg.

Use the right denominator

Enter final solution volume for molarity, rather than the volume of solvent added. For molality, enter solvent mass, excluding the dissolved substance. The tool does not convert molarity directly to molality without density data or check solubility, ionization or thermal expansion.

Calculate two concentrations from the same amount

Suppose 0.30 mol of solute is present in a solution with final volume 0.75 L. Its molarity is 0.30 / 0.75 = 0.40 mol/L. If preparation used 0.60 kg of solvent, its molality is 0.30 / 0.60 = 0.50 mol/kg. The results describe the same solution using different denominators. Changing the unit of one result cannot produce the other. Volume and solvent mass are separate example data.

If the solute amount is known as a mass, first convert it to moles using the correct molar mass. Purity and hydrate form can affect that conversion. For a final volume measured in millilitres, convert to litres or select the appropriate unit. For solvent mass measured in grams, remember the conversion to kilograms.

The result states concentration, not proof of complete dissolution. If undissolved material remains, do not treat the full weighed amount as dissolved moles. Assuming that component volumes always add can also change the calculated molarity. Record the temperature of the volume measurement and how the mole amount was established so that later comparisons remain meaningful.

Frequently asked questions

How do you calculate molarity?

In this example, dividing solute amount by final solution volume gives a molarity of 0.25 mol/L for 0.2 mol in 0.8 L. Use the final solution volume rather than the amount of solvent added, and note that solubility limits, ionization and thermal expansion are not checked.

Calculate with these values