How It Works
This calculator works in 4 steps.
Enter the starting molality
Type the value in mol/kg.
Enter the solvent mass present
Type the solvent mass, not the solution mass, in g or kg.
Enter the target molality
Type the lower target value in mol/kg.
Read the solvent to add
The calculator computes moles of solute, the new solvent mass, and the added solvent mass.
The Formula
The formula keeps the moles of solute constant and scales the solvent mass.
n = b₁ × W₁b₁ × W₁ = b₂ × W₂ΔW = W₂ − W₁- b₁ / b₂:
- Starting and target molality (mol/kg)
- W₁ / W₂:
- Starting and final solvent mass (kg)
- n:
- Moles of solute
A solution of 2 mol/kg NaCl in 1 kg water holds 2 mol NaCl. To reach 1 mol/kg, the solvent mass must be 2 kg, so add 1 kg (1,000 g) water.
Conversion Table
The conversion table shows the final solvent mass and solvent added for 6 molality dilutions.
| Starting molality | Solvent present | Target molality | Final solvent mass | Solvent to add |
|---|---|---|---|---|
| 2 mol/kg | 1,000 g | 1 mol/kg | 2,000 g | 1,000 g |
| 1 mol/kg | 500 g | 0.25 mol/kg | 2,000 g | 1,500 g |
| 5 mol/kg | 200 g | 1 mol/kg | 1,000 g | 800 g |
| 0.5 mol/kg | 1,000 g | 0.1 mol/kg | 5,000 g | 4,000 g |
| 3 mol/kg | 250 g | 0.75 mol/kg | 1,000 g | 750 g |
| 1 mol/kg | 2,000 g | 0.01 mol/kg | 200,000 g | 198,000 g |
What Is the Molality Dilution Calculator?
The Molality Dilution Calculator is a tool that dilutes solutions expressed in moles of solute per kilogram of solvent (mol/kg, m) by adding solvent mass. Molality depends on solvent mass, not solution volume, so the calculator works in grams rather than milliliters.
Molality stays constant with temperature, so physical chemistry uses it for freezing-point depression, boiling-point elevation, and osmolality. Dilution keeps the moles of solute fixed and adds solvent mass. The calculator returns the solvent mass to add and the new solvent total.
Common Uses
There are 5 main uses of this calculator.
Colligative property experiments
Students prepare solutions for freezing-point depression measurements.
Osmolality standards
Clinical labs prepare mOsm/kg calibrators by mass.
Thermodynamics research
Chemists report activity data on a molality basis.
Cryoprotectant mixes
Cryobiologists prepare glycerol and DMSO solutions by solvent mass.
Teaching labs
Instructors compare molarity and molality during temperature changes.