When scientists create a mixture, such as dissolving salt in water, they need a precise way to communicate its strength, known as concentration. Concentration is the quantification of the dissolved substance (solute) relative to the dissolving substance (solvent). Molality is one specific measure of concentration that chemists use to accurately describe the composition of a solution.
The Definition and Calculation of Molality
Molality, symbolized by a lowercase italicized \(m\), is a measure of concentration defined as the number of moles of solute divided by the mass of the solvent in kilograms. The resulting unit for molality is moles per kilogram (mol/kg), often referred to as “molal.” This definition uses mass in the denominator, which fundamentally differentiates it from other common concentration units.
To calculate molality, one first needs to determine the moles of solute. A mole is a standardized count of particles used by chemists because atoms and molecules are too small to count individually, allowing them to measure a specific number of particles by weighing a substance. Once the moles of solute are found, that value is divided by the mass of the solvent, which must be measured in kilograms, to find the molality.
Why Temperature Changes Demand Molality
The use of mass in the molality calculation provides a distinct scientific advantage, particularly when a solution is subjected to temperature changes. Most substances, including solvents, expand or contract when heated or cooled, causing their volume to change. Because mass is an intrinsic property of matter that does not change with temperature, a measurement based on mass, like molality, remains constant regardless of the solution’s temperature.
This temperature stability makes molality the preferred unit for experiments where temperature is intentionally varied or is difficult to control. Molality is used extensively when studying colligative properties, which are features of solutions that depend only on the number of solute particles, not their identity. These properties, such as freezing point depression and boiling point elevation, involve measuring temperature changes, requiring a concentration unit independent of temperature for accurate results.
Molality Versus Molarity
Molality is often confused with the more common concentration unit, molarity, represented by a capital \(M\). Molarity is calculated by dividing the moles of solute by the volume of the entire solution in liters. The primary distinction lies in the denominator: molality uses the mass of the solvent in kilograms, while molarity uses the volume of the entire solution in liters.
This difference in the denominator creates a significant practical contrast between the two measurements. Since molarity is volume-dependent, its value changes as the solution expands or contracts due to temperature fluctuations. Molality’s reliance on the unchanging mass of the solvent ensures its concentration value remains accurate across a range of temperatures.
For everyday laboratory work, molarity is frequently used due to the ease of measuring solution volume with glassware. However, when high precision is required, especially in physical chemistry or research involving temperature-sensitive measurements, molality is the superior choice. For very dilute aqueous solutions, the two values are often very close, but for concentrated or non-aqueous solutions, the difference becomes significant.