Molality Calculator
Calculate molality (mol solute per kg solvent) and estimate boiling-point elevation and freezing-point depression for aqueous solutions. For education only.
How to use this tool
- Enter solute mass, molar mass of solute and solvent mass in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your molality and the full breakdown beneath it.
Molality (m) = moles of solute ÷ kilograms of solvent. Unlike molarity, molality does not change with temperature. It is used in colligative property calculations: ΔT_b = K_b × m and ΔT_f = K_f × m (Kb = 0.512 °C·kg/mol, Kf = 1.853 °C·kg/mol for water).
For education only.
Formula
m = nsolute ÷ masssolvent(kg)
where nsolute = masssolute ÷ molar mass
Boiling-point elevation: ΔTb = Kb × m (Kb(water) = 0.512 °C·kg/mol)
Freezing-point depression: ΔTf = Kf × m (Kf(water) = 1.853 °C·kg/mol)
How it works
This calculator first converts the solute mass to moles (mass ÷ molar mass), then divides by the solvent mass in kilograms to obtain molality — the amount of solute per kilogram of solvent, a concentration unit that is independent of temperature.
Using the water ebullioscopic constant (Kb = 0.512 °C·kg/mol) and cryoscopic constant (Kf = 1.853 °C·kg/mol), the calculator estimates boiling-point elevation and freezing-point depression for aqueous solutions of non-electrolyte, non-volatile solutes (van't Hoff factor i = 1). Electrolytes or volatile solutes require adjusting the result by the appropriate i factor.
Worked example
Worked example: 18.015 g of water dissolved in 500 g of water
- Identify inputs: solute mass = 18.015 g, molar mass = 18.015 g/mol, solvent mass = 500.0 g.
- Calculate moles of solute: n = 18.015 ÷ 18.015 = 1.0 mol.
- Calculate molality: m = 1.0 mol ÷ (500.0 ÷ 1000 kg) = 2.0 mol/kg.
- Boiling-point elevation: ΔT_b = 0.512 × 2.0 = 1.024 °C.
- Freezing-point depression: ΔT_f = 1.853 × 2.0 = 3.706 °C.
Molality = 2.0 mol/kg; Boiling-point elevation = 1.024 °C; Freezing-point depression = 3.706 °C
Key terms
- Molality (m)
- The number of moles of solute per kilogram of solvent (not solution); unaffected by temperature changes.
- Boiling-point elevation
- The increase in a solvent's boiling point caused by dissolving a non-volatile solute; proportional to molality via K_b.
- Freezing-point depression
- The lowering of a solvent's freezing point due to a dissolved solute; proportional to molality via K_f.
- Ebullioscopic constant (K_b)
- The proportionality constant relating molality to boiling-point elevation; for water K_b = 0.512 °C·kg/mol.
- Cryoscopic constant (K_f)
- The proportionality constant relating molality to freezing-point depression; for water K_f = 1.853 °C·kg/mol.
Frequently asked questions
- How is molality different from molarity?
- Molarity (mol/L) is moles per litre of solution; molality (mol/kg) is moles per kilogram of solvent. Molality is temperature-independent because mass doesn't change with temperature, while volume does.
- What are colligative properties?
- Properties that depend only on the number of solute particles, not their identity: boiling-point elevation, freezing-point depression, osmotic pressure, vapour pressure lowering. Note: the shown Kb/Kf values assume non-electrolyte solutes.