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Depression of freezing point

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Depression of freezing point diagram
Solutions have lower freezing points that pure solvents. Freezing point depression is a colligative property of a solution derived from the Clausius-Clapeyron relation and Raoult's law. For ideal solutions, the cryoscopic constant can be calculated from theory.
Solvent freezing point
Cryogenic constant
Molality of solute
Dissocation number

Solution f.p.
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The dissocation number is the number of ions or parts that the solute breaks into when dissolved (e.g. 2 for NaCl, or 1 for sugar). Concentration is given in molality, but for weak solutions in water this approximates very closely to molarity.
While theory gives values for ideal cases, in reality cryoscopic constants are measured. Examples include: Acetic acid (3.90), Benzene (5.12), CCl4 (30), chloroform (4.68), ethanol (1.99) and water (1.86).