Molality Calculator
Choose what you want to find, enter the other two values and read off the molality, moles of solute and solvent mass.
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Formula
m = n ÷ mass of solvent (kg)
mis the molality in mol/kgnis the amount of solute in moles- the denominator is the solvent only, not the whole solution
Rearranged: n = m × kg of solvent and kg of solvent = n ÷ m. Grams are converted with 1 kg = 1000 g.
Worked example
0.5 mol of solute dissolved in 250 g of solvent:
- 250 g = 0.25 kg
m = 0.5 ÷ 0.25 =2 mol/kg
Reverse checks: 1.5 mol/kg in 2 kg of solvent holds 1.5 × 2 = 3 mol, and 3 mol at 1.5 mol/kg needs 3 ÷ 1.5 = 2 kg of solvent.
Frequently asked questions
What is molality?
Molality (m) is the amount of solute in moles divided by the mass of the solvent in kilograms. Dissolving 0.5 mol in 250 g (0.25 kg) of solvent gives 0.5 ÷ 0.25 = 2 mol/kg.
What is the difference between molality and molarity?
Molarity is moles per litre of solution and changes with temperature as the volume expands. Molality is moles per kilogram of solvent, so it does not change with temperature. That is why it is used for boiling and freezing point calculations.
How do I get molality from a mass of solute?
Divide the solute mass by its molar mass to get moles, then divide by the solvent mass in kg. For 10 g of NaCl (58.44 g/mol) in 500 g of water: 10 ÷ 58.44 = 0.1711 mol, and 0.1711 ÷ 0.5 = 0.342 mol/kg.
Is the denominator the solvent or the whole solution?
Only the solvent. If you weigh the finished solution, subtract the mass of the solute first. For 3 mol of solute at 1.5 mol/kg the solvent mass is 3 ÷ 1.5 = 2 kg.