Molality (b = n/m)

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.