Molality formula
Molality (m) is defined as moles of solute ÷ kilograms of solvent. Unlike molarity, it uses the mass of the solvent rather than the volume of the solution, so it does not change with temperature. Be careful: the denominator is solvent mass in kilograms, not total solution mass. For equivalents-based concentration, see the normality calculator.
How to use this calculator
Enter the moles of dissolved solute and the mass of the solvent in kilograms. If you only have grams, divide by 1000 first. The calculator divides moles by kilograms to give molality in mol/kg, and rejects a zero solvent mass.
If you start from a mass of solute rather than moles, divide that mass by the molar mass of the compound first. For water as the solvent there is a useful shortcut: 1 litre weighs almost exactly 1 kg at room temperature, so a litre of water can be entered as 1 kg. Molality uses the mass of solvent while molarity uses the volume of solution, which is the whole difference between them.
Worked example
Dissolve 0.5 mol of salt in 2 kg of water. Molality = 0.5 ÷ 2 = 0.25 m. Because molality is mass-based, this value stays the same whether the solution is hot or cold.
Read the result as the amount of solute that would accompany each kilogram of solvent: 0.25 mol per kg. For dilute aqueous solutions molality and molarity come out close together, but they diverge as the solution becomes concentrated or the solvent is not water, so quoting one when the other is required is a genuine source of error. Either way the numerator is an amount in moles, supplied by the moles calculator.
Frequently asked questions
- What is molality?
- Molality is the number of moles of solute per kilogram of solvent. It is written with a lowercase m and the units mol/kg.
- How is molality different from molarity?
- Molality uses kilograms of solvent, while molarity uses litres of solution. Molality is independent of temperature because mass does not expand or contract.
- What if I have grams of solvent?
- Convert grams to kilograms first by dividing by 1000, then enter the kilogram value.
- Why use the solvent mass, not the solution mass?
- Molality is defined relative to the solvent only, so you exclude the mass contributed by the dissolved solute.
- When is molality preferred over molarity?
- Use molality whenever the temperature changes during the work, as in boiling point elevation and freezing point depression calculations. Volumes expand when heated, so a molarity figure drifts, while a mass-based molality stays fixed.