Molality Calculator
Molality (m) measures the concentration of a solution as moles of solute per kilogram of solvent. Because it is defined in terms of mass rather than volume, molality is independent of temperature and pressure, making it the preferred concentration unit for colligative property calculations such as boiling point elevation, freezing point depression, and osmotic pressure. Enter the number of moles of solute and the mass of solvent in kilograms to find the molality of your solution.
Molality formula
m = moles of solute / mass of solvent (kg)
Where moles of solute is calculated by dividing the mass of the solute (g) by its molar mass (g/mol), and the solvent mass is in kilograms.
Understanding molality
- Molality is used in the boiling point elevation formula: delta T = i * Kb * m, where Kb is the ebullioscopic constant.
- For water, the boiling point elevation constant Kb is 0.512 degrees C per molal.
- Molality is used in the freezing point depression formula: delta T = i * Kf * m, where Kf for water is 1.86 degrees C per molal.
- A 1 m aqueous NaCl solution raises the boiling point of water by approximately 1.02 degrees C (van't Hoff factor i = 2).
- Molality cannot exceed the ratio of moles of solute to kg of solvent; there is no theoretical upper limit for miscible systems.
Frequently asked questions
What is molality?
Molality (m) is the number of moles of solute per kilogram of solvent. It differs from molarity (M), which is moles per liter of solution. Molality does not change with temperature because it is based on mass, not volume.
Why use molality instead of molarity?
Molality is preferred for colligative property calculations (boiling point elevation, freezing point depression, osmotic pressure) because it does not vary with temperature or pressure, unlike molarity.
How do I convert grams of solute to moles?
Divide the mass of solute in grams by the molar mass of the solute in g/mol. For example, 36.5 g of HCl (molar mass 36.5 g/mol) equals 1.00 mol.
What is the unit of molality?
The unit of molality is mol/kg, often written as lowercase m or molal. A 1 m solution contains 1 mol of solute per 1 kg of solvent.
Does molality include the mass of the solute?
No. Molality uses only the mass of the solvent (not the total solution mass). This is the key difference from mass percent, which uses the total solution mass.
Official sources
- IUPAC Gold Book: Molality definition.
- NIST Chemistry WebBook: NIST WebBook.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.