Molality Calculator

Molality is moles of solute per kilogram of solvent. Because it uses mass, it does not change with temperature.

A formula such as NaCl, H2SO4 or CuSO4·5H2O, or a molar mass in g/mol.
The solvent alone, not the whole solution.
Water: Kf 1.86, Kb 0.512.
Results update as you type.

Molality

0.199827 m

mol per kg of solvent

Moles of solute
0.0999134 mol
Freezing point depression
0.37168 KΔTf = Kf × m (non-electrolyte, i = 1)
Boiling point elevation
0.10231 KΔTb = Kb × m

For salts that split into ions, multiply the changes by the van ’t Hoff factor i (2 for NaCl).

About this calculator

With Kf and Kb (1.86 and 0.512 for water) it also gives the change in freezing and boiling points, as in Class 12 Solutions.

How to use it

  1. Enter the solute formula or molar mass.
  2. Enter the masses of solute and solvent.
  3. Read the molality and point changes.

The formula

m = moles of solute ÷ kg of solvent; ΔTf = Kf × m; ΔTb = Kb × m
Kf, Kb
cryoscopic and ebullioscopic constants

Worked example

18 g of glucose in 500 g of water

  1. Moles = 18 ÷ 180.16 = 0.0999.
  2. m = 0.0999 ÷ 0.5 = 0.200 m; freezing point falls by 0.372 K.

What the result means

For ionic solutes multiply ΔT by the van ’t Hoff factor i, about 2 for NaCl.

Assumptions

  • Dilute, ideal solutions; non-volatile solute.

Limitations

  • Concentrated solutions deviate from these simple formulas.

Frequently asked questions

Why use molality instead of molarity?

It does not depend on temperature.

What is Kf for water?

1.86 K kg/mol.

Do I use solvent or solution mass?

Solvent only.

Last reviewed on 6 October 2026. Found a mistake? Tell us.