Molality
Definition and meaning of Molality in chemistry.
Molality measures how concentrated a solute is in a solution. It is defined as the number of moles of solute dissolved per kilogram of solvent. The symbol for molality is a lowercase m, and its unit is mol/kg.
In more detail
Molality differs from molarity because it uses the mass of the solvent instead of the total volume of the solution. Volume can expand or contract as temperature changes, but mass does not change with temperature. This makes molality temperature-independent, unlike molarity.
Because of this stability, molality is especially useful for studying colligative properties, which depend only on the number of dissolved particles rather than their identity. Boiling point elevation and freezing point depression are two common colligative properties calculated using molality. To find molality, chemists measure the mass of solvent used, convert it to kilograms, and divide the moles of solute by that mass.
This calculation stays accurate even if the solution is heated or cooled during an experiment, since only the solvent's mass matters, not its volume. Molality is especially important in freezing point depression calculations. One example is figuring out how much salt lowers the freezing point of water on an icy road.
The relationship follows the equation ΔTf = Kf × m × i. Here, Kf is the solvent's freezing point depression constant, and i accounts for how many particles the solute breaks into. Because molality does not shift with temperature, it gives reliable, repeatable results across an entire experiment and across repeated trials.
Key facts
| Symbol | m |
|---|---|
| Unit | mol/kg |
| Definition | Moles of solute per kilogram of solvent |
| Temperature-dependent | No |
| Field | Physical Chemistry |
| Common use | Boiling point elevation, freezing point depression |
Dissolving 1.0 mole of sodium chloride (NaCl) in 1.0 kilogram of water produces a solution with a molality of 1.0 m. If this solution were heated, its molality would remain 1.0 m even though its volume would slightly increase.
Frequently asked questions
How is molality different from molarity?
Molality uses mass of solvent (kilograms) while molarity uses volume of solution (liters). Because molality is independent of temperature, it accurately predicts colligative properties like boiling point elevation and freezing point depression.
Why use molality instead of molarity?
Molality is preferred for studying temperature-dependent phenomena because its value does not change with temperature, whereas molarity changes as solution volume expands or contracts.
Can molality and molarity give the same numeric value?
For dilute aqueous solutions near room temperature, molality and molarity are often close in value, since one liter of water has a mass close to one kilogram. They diverge more in concentrated solutions.