Molality Calculator
By Santaji GadepatilinUpdated October 27, 2026
Molality is moles of solute per kilogram of solvent. Unlike molarity, it does not change with temperature, so it is used for colligative property calculations.
How to use the molality calculator
Enter the mass of solute, its molar mass and the mass of solvent (not solution). The moles of solute and molality are calculated.
Molality formula
b = nsolute ÷ msolvent (kg)
Worked example
11.69 g of NaCl (58.44 g/mol) in 0.5 kg of water:
- n = 11.69 ÷ 58.44 = 0.2 mol
- b = 0.2 ÷ 0.5 = 0.4 mol/kg
Molality vs molarity
| Molality (b) | Molarity (c) | |
|---|---|---|
| Definition | mol solute per kg solvent | mol solute per L solution |
| Changes with temperature? | No | Yes (volume changes) |
| Used for | Colligative properties | Everyday lab work |
For dilute aqueous solutions, molality and molarity are nearly equal.
Colligative properties
| Property | Formula | Constant for water |
|---|---|---|
| Freezing point depression | ΔT = i × Kf × b | Kf = 1.86 K·kg/mol |
| Boiling point elevation | ΔT = i × Kb × b | Kb = 0.512 K·kg/mol |
A 1 mol/kg NaCl solution (i ≈ 2) freezes at about −3.7 °C and boils at about 101.0 °C.
Road salt and antifreeze
Salt lowers the freezing point of water on roads. Ethylene glycol in car antifreeze works the same way; a 50% mixture protects down to about −37 °C, far below what the simple dilute formula predicts, because the solution is very concentrated.
Converting molarity to molality
b = c ÷ (ρ − c × M ÷ 1,000)
with ρ the solution density in kg/L and M in g/mol.
Freezing point depression
ΔTf = i × Kf × b. For water, Kf = 1.86 K·kg/mol; NaCl gives i ≈ 2. This solution freezes about 2 × 1.86 × 0.4 = 1.5 °C below 0 °C — the principle behind salting roads.
Frequently asked questions
What is molality?
Moles of solute per kilogram of solvent (mol/kg), written m.
How is molality different from molarity?
Molarity uses litres of solution; molality uses kilograms of solvent. Molality does not change with temperature, because mass does not.
Where is molality used?
In colligative properties — boiling point elevation, freezing point depression and osmotic pressure.
What is molality used for?
Calculating freezing point depression, boiling point elevation and other colligative properties, because it does not change with temperature.
Is molality the same as molarity?
No. Molality uses kilograms of solvent; molarity uses litres of solution. They are close for dilute water solutions.