Colligative Properties and Molality

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Molality[edit | edit source]

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[edit | edit source]

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