Molality is a concentration based on mass, not volume: moles of solute per kilogram of solvent. That choice has one big advantage — mass doesn't change when you heat or cool a solution, so a molality measured once stays correct at any temperature, unlike molarity which drifts as the volume expands. This temperature-stability is exactly why molality is the unit of choice for colligative properties like freezing-point depression and boiling-point elevation, where precise concentrations matter across a range of temperatures.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the definition of molal concentration.
The molality equations
The denominator is the mass of pure solvent in kilograms — a key difference from molarity, which divides by the solution's volume. If you start from a solute mass, first convert to moles with its molar mass, then divide by the solvent kilograms. Because both numerator (moles) and denominator (mass) are temperature-independent, the resulting molality is fixed for the life of the solution.
Worked example — a molal solution
Scenario: You dissolve 0.5 mol of solute in 0.25 kg of solvent. What is the molality?
The solution is 2 molal. Spread the same 0.5 mol through 1 kg of solvent instead and the molality drops to 0.5 m — more solvent, more dilute. Note that if you had quoted this as a molarity it would shift slightly when warmed, because the 0.25 kg of solvent occupies more volume hot than cold; the molality of 2 mol/kg, by contrast, stays exactly 2 at any temperature.
Frequently Asked Questions
m = moles solute ÷ kg solvent. 0.5 mol in 0.25 kg = 2 mol/kg (2 molal). Solvent mass, not solution.
Molarity = mol/L of solution (volume, temperature-dependent); molality = mol/kg of solvent (mass, temperature-stable).
It doesn't change with temperature, so it's used for colligative properties (freezing/boiling point, osmosis).
moles = grams ÷ molar mass. 18 g of M=180 is 0.1 mol; in 0.5 kg solvent = 0.2 mol/kg.
Roughly, for dilute aqueous solutions (1 L water ≈ 1 kg). They diverge when concentrated, non-aqueous or hot.