Calculate molality (m) in mol/kg from solute and solvent
A molality calculator computes the molality (m) of a solution — the number of moles of solute dissolved per kilogram of solvent. Unlike molarity, molality is based on the mass of the solvent rather than the volume of the solution, so it does not change with temperature. This makes molality the preferred concentration unit for colligative-property calculations such as boiling-point elevation and freezing-point depression.
The molality of a solution is given by:
Dissolve 0.5 mol of NaCl in 250 g (0.25 kg) of water. Molality = 0.5 ÷ 0.25 = 2.0 mol/kg. The solution is 2.0 molal.
Dissolve 90 g of glucose (molar mass 180 g/mol) in 500 g of water. Moles of solute = 90 ÷ 180 = 0.5 mol; solvent = 0.5 kg. Molality = 0.5 ÷ 0.5 = 1.0 mol/kg.
Molality (m) is moles of solute per kilogram of solvent, while molarity (M) is moles of solute per litre of solution. Molality uses mass, so it does not change with temperature; molarity uses volume, which expands or contracts as temperature changes.
Molality is expressed in moles per kilogram (mol/kg), often written as "molal" or with a lowercase italic "m". A 1 molal solution contains one mole of solute per kilogram of solvent.
First convert the solute mass to moles by dividing by its molar mass, then divide the result by the mass of the solvent in kilograms.
Colligative properties such as freezing-point depression and boiling-point elevation depend on the amount of solute relative to solvent mass. Because molality is temperature-independent, it gives consistent results across temperature changes, unlike molarity.
Use only the mass of the solvent (for example, the water), not the combined mass of solute plus solvent. This is a common mistake that leads to slightly low molality values.