Molality (b = n/m)
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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Grade 12Grade 12 Chemistry
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Molality (b = n/m) explained
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
- = Molality (mol/kg)
- = Amount of solute (mol)
- = Mass of solvent (kg)
Missing one of these? Work it out first, then come back
- Molality — Boiling-Point Elevation, Freezing-Point Depression
- Amount of solute — Molarity (C = n/V), Mole Fraction
- Mass of solvent — Density, Sensible Heat (Q = mcΔT)