Molality Calculator
Molality calculator: moles of solute per kilogram of solvent (mol/kg), used for boiling point elevation and freezing point depression.
b = msoluteM · msolvent
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
Molality b is the amount of solute divided by the mass of the solvent, b = m/(M·msolvent), in mol/kg. Because it uses masses, not volumes, it does not change with temperature.
Molality is the concentration used in colligative properties such as freezing point depression (ΔT = Kf·b·i) and boiling point elevation.
Formula
b = msoluteM · msolvent
| Symbol | Quantity | Units |
|---|---|---|
| m | Mass of solute | kg, g, mg, µg, t, lb, oz, st, US ton, ct, Da |
| M | Molar mass | g/mol, kg/mol |
| ms | Mass of solvent | kg, g, mg, µg, t, lb, oz, st, US ton, ct, Da |
| b | Molality | mol/kg, mmol/kg |
Worked example
Given: m = 18 g · M = 180.156 g/mol · ms = 0.5 kg
b = mM · ms
Result: b = 0.199827 mol/kg
Frequently asked questions
Why is molality used for freezing point depression?
The effect depends on the ratio of solute particles to solvent mass, and molality expresses exactly that, independent of temperature.
Can I convert molarity to molality?
Only if you know the solution density: b = c / (ρ − c·M), with consistent units.