Colligative Properties and Molality

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Molality

Molality is expressed as moles/mass (mol/kg) in contrast with molarity which is expressed as moles/volume (mol/L). Molality is useful in situations which involve a large temperature or pressure change which can drastically change the volume, changing the molarity, but not the molality of the solution. As molality is a more accurate measure of solutes in solution in dynamic conditions, it is often used in comparing and determining colligative properties.

Colligative Properties

Colligative properties are properties of solutions that are determined solely by the number of particles dissolved rather than their nature. These properties include:

Vapor Pressure

Boiling Point

Boiling point increases with increasing molality of a solution according to the following equation: Boiling Point of solution = Boiling Point of solvent + Tb Tb = m * Kb * i, where m = molality, Kb = ebullioscopic constant (varies for each solvent), and i = Van't Hoff factor The Van't Hoff factor is equal to the number of unique solutes and/or ions in the solution.

Melting Point

Melting point decreases with increasing molality of a solution according to the following equation: Melting Point of solution = Melting Point of solvent + Tf Tf = m * Kf * i, where Kf = cyroscopic constant (varies for each solvent)

Osmotic Pressure