Molalita

Molalita is a measure of the concentration of a solute in a solution, defined as the amount of substance of solute in moles per kilogram of solvent. The unit is mole per kilogram (mol/kg), and the symbol m is commonly used. Molalita emphasizes the mass of the solvent rather than the total solution volume, which distinguishes it from molarity, a more familiar concentration measure based on liters of solution. Because it relies on mass, molalita is largely insensitive to temperature changes, making it particularly useful in thermodynamic calculations.

Calculation of molalita is straightforward: m = n_soluto / m_solvante, where n_soluto is the amount of solute in moles and m_solvante is the solvent mass in kilograms. For example, dissolving 2.0 moles of solute in 0.500 kilograms of solvent yields a molalita of 4.0 mol/kg.

Molalita has important applications in physical chemistry, especially in the calculation of colligative properties. Changes in boiling point and freezing point depend on molalita through formulas such as ΔTb = i Kb m and ΔTf = i Kf m, where i is the van't Hoff factor and Kb and Kf are solvent constants. It is a useful parameter in experiments where temperature varies or when precise control over solvent mass is available.

Limitations include potential inaccuracies if substantial solvent mass changes during dissolution or if strong solute–solvent interactions alter the effective solvent mass. Accurate determination requires careful separation of solvent mass from the total solution mass.

See also: Molality, Colligative properties, Molarity, Mole fraction.