Quick answer
Molality is moles of solute per kilogram of solvent. 0.5 mol of solute in 0.25 kg of solvent is 2 mol/kg.
Enter the moles of solute and the mass of solvent. The calculator returns the molality.
Related tools: Mole Calculator: Convert between mass, moles and number of particles. Percent Yield Calculator: Percent yield of a reaction. Electron Configuration Calculator: Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons.
How to use the Molality Calculator
- Enter moles of solute (mol) (above 0).
- Enter mass of solvent (kg) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Moles of solute | A number, in mol | > 0 |
| Mass of solvent | A number, in kg | > 0 |
Formula and method
Formula
molality = moles of solute ÷ kilograms of solvent
IEEE-754 double precision; shown to the decimals indicated.
Uses solvent mass, not solution volume, so it does not change with temperature.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Moles of solute = 0.5; Mass of solvent = 0.25.
| Result | Value |
|---|---|
| Molality | 2 mol/kg |
Example 2
Inputs: Moles of solute = 0.1; Mass of solvent = 0.5.
| Result | Value |
|---|---|
| Molality | 0.2 mol/kg |
The first line is the headline result (molality). IEEE-754 double precision; shown to the decimals indicated.
Molality versus molarity
Molality uses the mass of the solvent, not the volume of solution, so it doesn't change with temperature. It is the right measure for boiling-point elevation and freezing-point depression: ΔT = K × m.
Details
The solvent mass excludes the solute. For water, 1 kg is about 1 L. A 2 mol/kg solution of sodium chloride would lower the freezing point of water by about 2 × 1.86 × 2 = 7.4 °C, counting the van 't Hoff factor.
How the result changes with moles of solute
Holding every other input at the first worked example, this table changes only moles of solute. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Moles of solute | Molality |
|---|---|
| 0.25 | 1 mol/kg |
| 0.5 | 2 mol/kg |
| 1 | 4 mol/kg |
| 2 | 8 mol/kg |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Moles of solute: entering "0" shows "Moles of solute must be greater than 0." beside that field and no result.
- Mass of solvent: entering "0" shows "Mass of solvent must be greater than 0." beside that field and no result.
Choosing between the Molality Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Mole Calculator](/science/mole/) | start from and molar mass | Amount |
| [Percent Yield Calculator](/science/percent-yield/) | actual yield and theoretical yield | Percent yield |
| [Electron Configuration Calculator](/science/electron-configuration/) | element and charge | Electron configuration of H |
| [Dilution Calculator](/science/dilution-ratio/) | solve for | Final volume V₂ |
| [Molarity Calculator](/science/molarity-calculator/) | solute mass, molar mass and solution volume | Molarity |
Background: chemistry calculations
Most basic chemistry calculations chain three relations: moles n = mass ÷ molar mass, molarity M = n ÷ volume in litres, and dilution C₁V₁ = C₂V₂. Keep units consistent: grams with g/mol, litres with mol/L.
Molar masses come from standard atomic weights, which differ slightly between tables and with the isotopic make-up of a real sample.
Assumptions and limitations
- Solvent mass excludes the solute
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for moles of solute and mass of solvent to confirm this.
Frequently asked questions
What is the unit?
Molality is in mol/kg of solvent; the example's 0.5 mol in 0.25 kg gives 2 mol/kg.
Why use molality?
It is temperature independent.
How is it different from molarity?
Molarity uses solution volume.
What does the Molality Calculator calculate?
Molality of a solution. It returns Molality.
What formula does the Molality Calculator use?
molality = moles of solute ÷ kilograms of solvent. Uses solvent mass, not solution volume, so it does not change with temperature.
What do I need to enter in the molality Calculator?
You enter Moles of solute (mol) and Mass of solvent (kg).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Solvent mass excludes the solute.
What is an example of a molality result?
With moles of solute 0.5 and mass of solvent 0.25, the molality is 2 mol/kg.
Related calculators
- Mole Calculator — Convert between mass, moles and number of particles.
- Percent Yield Calculator — Percent yield of a reaction.
- Electron Configuration Calculator — Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons.
- Molarity Calculator — Concentration of a solution from solute mass, molar mass and volume.
- Average Atomic Mass Calculator — Weighted average of isotope masses.
- pH Calculator — pH, pOH and ion concentrations at 25 °C.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.