Molality Calculator
Moles and molality from grams, molar mass, and solvent.
What it does
Molality counts moles of solute against kilograms of solvent, so heat and dilution by temperature never move it. Colligative properties like freezing point drop and boiling point rise are built on it for exactly that reason.
The tool above takes solute grams, molar mass, and solvent kilograms. It returns moles with molality beside them, keeping the solvent mass visible so solution mass mistakes stand out.
The defaults dissolve 29.22 g of table salt (molar mass 58.44) in 0.25 kg of water: 0.5 mol at 2 m. OpenStax works the same line for ammonia, 0.85 g in 0.125 kg giving 0.4 m, and for ethylene glycol at 17.9 m.
The definition matches OpenStax Chemistry colligative properties with NIST SI amount of substance behind the mole. Both state molality as amount of solute in moles per kilogram of solvent, written with a lowercase m.
How to use
- Enter solute grams on the balance.
- Enter molar mass in g/mol.
- Enter solvent kilograms, solute excluded.
- Read moles first.
- Read molality beside them.
- Compare with molarity only at stated temperature.
Formula
- m
- Molality in mol/kg, lowercase to differ from M.
- mol
- Amount of solute, grams divided by g/mol.
- kg
- Mass of solvent only, never solution.
Volume never enters, so warming or cooling the flask leaves molality alone while molarity drifts. Near solubility limits and with strong electrolytes, activity corrections join the story for real colligative work.
Worked example
Example: 29.22 g NaCl in 0.25 kg water
Defaults: 29.22 g, molar mass 58.44, solvent 0.25 kg.
- Moles: 29.22 / 58.44 = 0.5 mol.
- Molality: 0.5 / 0.25 = 2 m.
- Solvent: 0.25 kg = 250 g.
- Tool shows 2 m, 0.5 mol, 250 g solvent.
- Heat the flask and m stays 2 while M drifts.
Reference table
| Mix | Math | Result |
|---|---|---|
| 58.44 g NaCl in 1 kg | 1 mol / 1 kg | 1 m |
| 10 g NaOH in 0.5 kg | 0.25 mol / 0.5 kg | 0.5 m |
| 0.85 g ammonia in 0.125 kg | 0.0499 mol / 0.125 kg | 0.4 m |
| 2222.1 g glycol in 2 kg | 35.8 mol / 2 kg | 17.9 m |
Limits
- Needs full dissolving; undissolved solid breaks the line.
- Strong electrolytes need activity, not raw m, for exact work.
- Past solubility the extra solid simply sits out.
- Trace water in hygroscopic solutes skews grams.
Common mistakes
- Dividing by solution mass instead of solvent.
- Writing M when the work needs m.
- Using milliliters where kilograms belong.
- Forgetting waters of hydration in molar mass.
- Comparing m and M across temperatures.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 g NaOH (M 40) in 0.5 kg?
0.25 mol, 0.5 m.
0.85 g ammonia (M 17.03) in 0.125 kg?
0.0499 mol, 0.4 m.
Why unchanged when heated?
Mass basis: neither moles nor kg of solvent move with temperature.
Questions
What is molality?
Moles of solute per kilogram of solvent, written lowercase m.
How is it different from molarity?
Molarity divides by liters of solution and drifts with temperature; molality does not.
Solvent or solution mass?
Solvent only; adding solute mass is the classic error.
What units result?
mol/kg, often just called molal.
Can molality be zero?
Yes, with no solute; negative values are meaningless.
When does it break down?
Past solubility and in concentrated electrolytes without activity corrections.