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Molality (b = n/m)

b=nmsolventb = \frac{n}{m_{\text{solvent}}}
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Worked example: 0.5 mol in 250 g water → b = 2 mol/kg. Press Try an example to run it live, then adjust anything.

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Where Molality (b = n/m) fits

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Freezing-Point Depression with the van 't Hoff FactorBoiling-Point ElevationFreezing-Point Depression
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See also

Freezing-Point Depression with the van 't Hoff Factor: The main thing molality is for: the colligative laws are written in it because kilograms of solvent do not change with temperature.Boiling-Point Elevation: Both use molality.Freezing-Point Depression: Both use molality.
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Molality (b = n/m) explained

msolventnb

Molality looks like molarity's twin but divides by the mass of solvent, not the volume of solution, and that swap is the whole point. Dissolve 0.500 mol of glucose in 250 g (0.250 kg) of water and the molality is 0.500/0.250 = 2.00 mol/kg, a value that stays exactly 2.00 whether the flask sits in an ice bath or on a hot plate, because mass does not expand with temperature the way volume does.

That temperature-independence is why colligative-property formulas, boiling-point elevation and freezing-point depression, are written in terms of molality. For dilute aqueous solutions the two scales nearly coincide (1 L of water is 1 kg), but in concentrated solutions or non-aqueous solvents they diverge sharply, and mixing them up is a classic exam trap.

Molality (b = n/m) formula

b=nmsolventb = \frac{n}{m_{\text{solvent}}}
Where
  • bb= Molality (mol/kg)
  • nn= Amount of solute (mol)
  • msolventm_{\text{solvent}}= Mass of solvent (kg)
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Ways to express concentration

6 formulasMolarity, molality, mass percent, mole fraction, normality and the dilution equation — six answers to “how much solute is in there?”

Revised 16 October 2026
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formula.expert. (2026, October 16). Molality (b = n/m) Formula & Calculator. https://formula.expert/solvers/molality-definition

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