Molality Calculator
Find the molality of a solution — moles of solute per kilogram of solvent. Enter the solute mass in grams, its molar mass, and the solvent mass in grams or kilograms to get molality (m) and the moles dissolved.
Example: with Solute mass (g) 20 · Molar mass of solute (g/mol) 58.44 · Solvent amount 250 · Solvent unit g (grams) → Molality: 1.3689 mol/kg (m).
- Moles of solute0.3422 mol solute
- Concentration bandConcentrated — expect measurable freezing- and boiling-point shifts
Computed by the calculator below using its default values. Change any input to see your own numbers.
Molality m = moles of solute ÷ kilograms of solvent. Unlike molarity it uses solvent mass, not solution volume, so it does not drift with temperature.
Molality versus molarity
Molality counts moles of solute per kilogram of solvent, while molarity counts moles per liter of solution. The difference matters because volume expands when a solution warms up, so molarity slowly falls as temperature rises. Mass never changes, so molality stays fixed no matter the temperature — which is why it is the concentration of choice for freezing-point depression and boiling-point elevation problems.
To get the moles, divide the solute mass by its molar mass: 20 g of table salt (58.44 g/mol) is 0.342 mol. Dividing by the solvent mass in kilograms — 0.250 kg here — gives 1.37 mol/kg. Note that you weigh the solvent, not the finished solution.
How it’s calculated
Molality m = n_solute / kg_solvent, where n_solute = solute mass (g) ÷ molar mass (g/mol). Solvent mass entered in grams is divided by 1000 to reach kilograms. Results are rounded to four decimals.
The mass you enter is the solvent alone, not the total solution. Molar mass must be the correct formula mass of the solute for the mole count to be right.
Molality of familiar solutions
| Solution | Approximate molality |
|---|---|
| Isotonic saline (0.9 g NaCl per 100 g water) | ≈ 0.15 m |
| Seawater (≈ 35 g salt per kg water) | ≈ 0.60 m |
| 1-molal sugar syrup (342 g sucrose per kg water) | 1.0 m |
| Saturated table salt (≈ 360 g per kg water) | ≈ 6.1 m |
Computed as mol solute ÷ kg solvent from typical compositions; rounded.
Common mistakes
- Dividing by the solution volume instead of the solvent mass — that gives molarity, not molality.
- Using solvent volume in liters as if it were kilograms; only exactly true for water near room temperature.
- Entering the wrong molar mass, which scales the mole count and the answer directly.
- Weighing the whole solution rather than just the solvent.
Frequently asked questions
What is the formula for molality?
Molality m = moles of solute ÷ kilograms of solvent. Find moles by dividing the solute mass by its molar mass, then divide by the solvent mass in kilograms.
How is molality different from molarity?
Molality uses the mass of solvent in kilograms; molarity uses the volume of solution in liters. Molality does not change with temperature because mass is fixed, while molarity does.
What units does molality use?
Moles per kilogram, written mol/kg or m (lowercase italic). A 1 m solution has one mole of solute per kilogram of solvent.
Why use molality for freezing-point problems?
Freezing-point depression and boiling-point elevation depend on ΔT = K × m. Because those experiments change temperature, molality is used since it stays constant while volume-based molarity would not.