Molality is the moles of solute divided by the mass of solvent in kilograms, written mol/kg or m. Dissolve 18 g of glucose (180.16 g/mol) in 500 g of water: 18 ÷ 180.16 = 0.0999 mol, and 0.0999 ÷ 0.500 kg = 0.1998 mol/kg. Unlike molarity, it does not change when the solution warms or cools.
Molality matters wherever temperature moves: freezing-point depression, boiling-point elevation and osmotic work all use mol/kg because a mass of solvent does not expand the way a volume of solution does. Students preparing colligative-property problems and lab staff weighing out reference solutions both need it. Enter the solute either as a mass with its molar mass or directly as moles, then the solvent mass in grams or kilograms. The result shows molality to four decimals, the moles of solute, the solvent mass in kilograms, the millimolal value and, when a solute mass is known, the mass percent of the mixture. One limit: the denominator is the solvent alone, so if you only know the total mass of solution, subtract the solute mass first.
Pick mass and molar mass if you weighed the solute, or amount in moles if you already know n.
Type the mass in grams and the molar mass in g/mol, or the amount in mol.
Type the solvent mass and set the unit beside it to g or kg.
Molality appears in mol/kg with the working listed beneath it, including the mole calculation.
Result Molality (mol/kg): 1
Moles of solute: 1
Solvent mass (kg): 1
Millimolal (mmol/kg): 1,000
Solute mass percent: 5.521%
One mole of sodium chloride in one kilogram of water is the textbook 1 molal solution.
Result Molality (mol/kg): 0.1998
Moles of solute: 0.099911
Solvent mass (kg): 0.5
Millimolal (mmol/kg): 199.82
Solute mass percent: 3.475%
18 g ÷ 180.16 g/mol is 0.0999 mol; divided by 0.5 kg of water that gives about 0.1998 mol/kg.
Result Molality (mol/kg): 0.5
Moles of solute: 0.25
Solvent mass (kg): 0.5
Millimolal (mmol/kg): 500
Solute mass percent: Needs the solute mass
When the amount is already in moles, molality is a single division: 0.25 ÷ 0.5 = 0.5 mol/kg.
b = n ÷ m_solvent, with n = m_solute ÷ M
bnm_soluteMm_solventIUPAC defines molality as the amount of solute divided by the mass of solvent. The calculator converts the solute mass to moles with its molar mass, converts a solvent mass given in grams to kilograms by dividing by 1000, then divides. Mass percent uses solute mass over solute plus solvent mass.
Molarity divides moles of solute by litres of the whole solution, while molality divides by kilograms of solvent only. Because volume changes with temperature and mass does not, molality stays fixed as a solution warms. For dilute water solutions near room temperature the two numbers are close, since a litre of water weighs about one kilogram.
The relation ΔT = K × b uses molality because the cryoscopic and ebullioscopic constants are defined per kilogram of solvent. For water, Kf is 1.86 °C·kg/mol, so a 0.5 mol/kg solution of a non-electrolyte freezes about 0.93 °C below pure water.
A lowercase m, read as molal, is shorthand for mol/kg. A 2 m sucrose solution holds 2 moles of sucrose per kilogram of water. IUPAC prefers writing mol/kg in full to avoid confusion with metres or with molar (M).
Yes. The definition only needs the solvent mass, so ethanol, benzene or any other liquid works the same way. If you measured the solvent by volume, multiply by its density first to get the mass.
Subtract the solute mass from the solution mass to get the solvent mass, then enter that. For example, 110 g of solution containing 10 g of salt has 100 g of solvent, or 0.100 kg.
Add the standard atomic masses of every atom in the formula. Sodium chloride is 22.99 + 35.45 = 58.44 g/mol, and glucose, C6H12O6, is 180.16 g/mol. For a hydrate, include the water molecules in the formula.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 17). Molality Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/solutions-and-concentration/molality-calculator/
Tools Directory Editorial Team. “Molality Calculator.” Tools Directory, 17 Oct. 2026, https://tools.milunir.com/chemistry/solutions-and-concentration/molality-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Molality Calculator.” Tools Directory. Last modified October 17, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/solutions-and-concentration/molality-calculator/.
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