Molality

In-depth explanation of Molality

Molality is the concentration of a solution expressed as moles of solute per kilogram of solvent, m = mol solute / kg solvent. Because it uses the mass of solvent rather than the volume of solution, molality does not change with temperature. This makes it the preferred unit for colligative property calculations such as boiling point elevation and freezing point depression. Molality is also convenient when the solvent can be easily weighed in the laboratory. It should not be confused with molarity, which depends on the volume of the solution.

Going deeper: Molality

Key ideas: Formula: m = mol solute / kg solvent. To convert from molarity, use density: mass of solution per litre minus the mass of solute gives the solvent mass. For dilute aqueous solutions molality is close to molarity. Used in colligative formulas, with Kf and Kb in units of C kg/mol.

Common mistakes and connections: Students use kg of solution instead of kg of solvent. Another frequent slip is leaving grams of solvent unconverted. Molality does not change with temperature, but cannot be converted to molarity without a density. Do not confuse molal with molar in notation. Links to boiling point elevation, freezing point depression and mole fraction.

Visualization of Molality

Molality is moles of solute per kilogram of solvent; it does not change with temperature, so it is used in colligative property work. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Dissolving 1.0 mol of solute in 1.0 kg of water gives a solution with a molality of exactly 1.0 mol/kg.

Where Molality fits in States of Matter & Solutions

Molality is topic 77 of 109 in States of Matter & Solutions.

Related topics

• Saturated Unsaturated SupersaturatedBeginner
• Like Dissolves LikeBeginner
• MolarityBeginner
• Mass PercentBeginner
• Parts Per MillionBeginner
• NormalityIntermediate

Browse all 109 topics in States of Matter & Solutions