What molality measures
Molality expresses concentration as moles of solute per kilogram of solvent. Unlike molarity (which uses volume), molality is temperature-independent because mass doesn’t change when a solution expands or contracts with temperature. This makes it the preferred concentration unit for colligative property calculations like boiling point elevation and freezing point depression.
The most common error is confusing solvent mass with solution mass. Molality uses only the mass of the solvent (the substance doing the dissolving), not the total mass of solvent plus solute combined.
How to Calculate Molality (step by step)
Step One: Calculate moles of solute
Divide the mass of solute by its molar mass.
moles of solute = mass of solute (g) ÷ molar mass (g/mol)
For 58.44 g of NaCl (molar mass 58.44 g/mol): 58.44 ÷ 58.44 = 1.000 mol.
Step Two: Convert solvent mass to kilograms
Divide the solvent mass in grams by 1000.
solvent mass (kg) = solvent mass (g) ÷ 1000
For 1000 g of water: 1000 ÷ 1000 = 1.000 kg.
Step Three: Divide moles by kilograms
Divide moles of solute by mass of solvent in kg.
molality = moles of solute ÷ mass of solvent (kg)
For this example: 1.000 ÷ 1.000 = 1.000 mol/kg (a 1 molal NaCl solution).