What is molality and how does it differ from molarity?

Molality (m) is the number of moles of solute per kilogram of solvent. Molarity (M) is the number of moles of solute per litre of solution. Molality does not change with temperature because it is based on mass rather than volume, making it more reliable for colligative properties like boiling point elevation and freezing point depression.

What is the formula for molality?

Molality m = moles of solute divided by mass of solvent in kilograms. For example, dissolving 0.5 mol of NaCl in 0.25 kg of water gives a molality of 2.0 mol/kg.

Why is molality used in boiling point and freezing point calculations?

Boiling point elevation and freezing point depression are colligative properties that depend on the number of solute particles relative to solvent molecules, not the total volume of solution. Because volume changes with temperature but mass does not, molality gives a more consistent measure for these calculations.

Molality Calculator

Molality is a concentration unit that expresses the amount of solute dissolved in a fixed mass of solvent, specifically the number of moles of solute per kilogram of solvent. Unlike molarity, which is moles per litre of solution, molality does not change as temperature changes because mass is unaffected by thermal expansion. This makes molality the preferred concentration measure for colligative properties such as boiling point elevation, freezing point depression and osmotic pressure, where the effect depends on the ratio of solute particles to solvent molecules rather than the total volume of the mixture. The formula is m equals moles of solute divided by mass of solvent in kilograms. This calculator has three modes. The first calculates molality from moles and solvent mass in kilograms. The second finds the required moles of solute to reach a target molality for a given solvent mass. The third finds the solvent mass needed to reach a target molality for a given number of moles. To find moles from a mass and molar mass, you can use the molar mass calculator first and then enter the mole value here. The calculator is used in chemistry courses from secondary school through to university, and in any application involving dissolved substances such as antifreeze solutions, electrochemistry and food science. Solvent mass should be the mass of pure solvent only, not the total solution mass.

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Solve for

Moles of solute (mol)

mol

Mass of solvent (kg)

Target molality (mol/kg) - for moles or mass mode

mol/kg

2.00 mol/kg

molality

Moles of solute0.500 mol

Solvent mass0.250 kg

Enter the mass of pure solvent only (not the total solution mass). For water: 1 L water = approximately 1 kg at 25 degrees C.

How it works

The molality formula is m = moles / kg_solvent. To find moles: moles = m x kg_solvent. To find solvent mass: kg_solvent = moles / m. Molality is temperature-independent because it uses mass (kg) rather than volume (L), unlike molarity. For colligative properties: boiling point elevation deltaT_b = K_b x m x i, where K_b is the ebullioscopic constant of the solvent (0.512 degC kg/mol for water) and i is the van't Hoff factor.

Worked example

You dissolve 0.5 mol of a solute in 0.25 kg of water. Molality m = 0.5 divided by 0.25 = 2.00 mol/kg. These match the default values pre-filled above. In water (K_b = 0.512 degC kg/mol), this solution would elevate the boiling point by 0.512 x 2.00 = 1.024 degC (for a non-electrolyte with i = 1).

Related calculators

Molar Mass Calculator: find the molar mass of a compound to convert grams to moles.
Ideal Gas Law Calculator: solve PV = nRT for moles, pressure, volume or temperature.
pH Calculator: calculate pH from hydrogen ion concentration.
Percent Error Calculator: compare an experimental concentration with the expected value.

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