Molality Formula and Molality Calculator
Molality Formula and Molality Calculator
Calculate molality, mass of solute or mass of solvent. With step-by-step working, the difference from molarity and a colligative-properties example.
What is molality?
Molality (m) is the number of moles of solute per kilogram of solvent. Its unit is mol/kg, written m (molal). A 0.2 m glucose solution contains 0.2 mol of glucose for every 1 kg of water.
Because it is based on mass rather than volume, molality does not change with temperature. That is why it is used for colligative properties such as boiling-point elevation and freezing-point depression.
Molality formula
Worked examples
Example 1: calculating molality
9.008 g of glucose (Mr = 180.16 g/mol) is dissolved in 250 g of water. What is the molality?
- n = 9.008 g ÷ 180.16 g/mol = 0.05 mol
- p = 250 g = 0.25 kg
- m = 0.05 mol ÷ 0.25 kg = 0.2 mol/kg
Answer: 0.2 m
Example 2: mass of solute
How many grams of urea (Mr = 60.06 g/mol) must be dissolved in 200 g of water to give 0.5 m?
- p = 200 g = 0.2 kg
- n = m × p = 0.5 mol/kg × 0.2 kg = 0.1 mol
- mass = 0.1 mol × 60.06 g/mol = 6.006 g
Answer: 6.006 g of urea.
Example 3: molality and freezing-point depression
A 0.2 m aqueous glucose solution. What is its freezing point? (Kf of water = 1.86 °C/m, glucose is a non-electrolyte)
- ΔTf = Kf × m = 1.86 °C/m × 0.2 m = 0.372 °C
- Freezing point = 0 °C − 0.372 °C
Answer: −0.372 °C
Molality vs molarity
| Molality (m) | Molarity (M) | |
|---|---|---|
| Divided by | solvent mass (kg) | solution volume (L) |
| Unit | mol/kg | mol/L |
| Temperature dependent | no | yes |
| Used for | colligative properties | preparing solutions, titration |
For dilute aqueous solutions the two values are almost equal, because 1 litre of water weighs about 1 kg. For concentrated solutions or non-aqueous solvents they can differ a lot. See also the molarity formula.
Frequently asked questions
What is the unit of molality?
Molality is expressed in mol/kg, written m (molal).
Why does molality not depend on temperature?
It is based on the mass of solvent, and mass does not change with temperature. Molarity uses volume, which expands or contracts.
Is the mass of solvent the same as the mass of solution?
No. Mass of solution = mass of solute + mass of solvent. Molality uses the solvent mass only.
When should I use molality?
For colligative properties (boiling-point elevation, freezing-point depression, osmotic pressure of concentrated solutions) and when the temperature varies.
How do I convert molality to molarity?
You need the density of the solution. Find the solution mass from solute plus solvent, convert it to volume with the density, then divide the moles by that volume.
Prepare solutions with analytical-grade reagents
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Other formulas & calculators: molarity, mole, normality, dilution, titration, pH, buffer, molar mass, ppm, mole fraction, mass percent, periodic table, chemical MSDS.