Freezing-point lowering, Determination

Freezing point lowering (or other colligative properties) can be used to determine the extent of dissociation of a weak electrolyte in water. The procedure followed is illustrated in Example 10.11. 

Freezing point methods are often applied to the measurement of activities of electrolytes in dilute aqueous solution because the freezing point lowering, 6= T — T, can be determined with high accuracy, and the solute does not dissolve in the solid to any appreciable extent. Equations can be derivedgg relating a to 9 instead of T and T. The detailed expressions can be found in the literature.16... 

Raoult s law works for small polymers as well as small molecules. Determination of M is based for both ebulliometry (boiling point elevation) and cryometry (freezing point lowering) on the Clausius-Clapeyron equation ... 

Na2C03 (liq.). Sackur1 determined the heat of fusion from freezing point lowering data. 

We can determine the molality by looking at the freezing-point lowering, 0.52°C. 

Figure 1.67 shows a schematic of a simple, but effective set-up for cryoscopy, the method for the measurement of the freezing point lowering. Cryoscopy is perhaps the easiest of the molar mass determinations. The main prerequisites are a good temperature control and uniformity, corrections for the common supercooling observed on crystallization, and the usual extrapolation to infinite dilution. The thermodynamic equations are derived in Sect. 2.2.5, together with the equations needed for the ebulliometry. 

The freezing point (Tj) of a DES determines directly whether this DES is feasible or not to be used as a real solvent. All the DESs reported to date have a freezing point lower than 150°C, but those that remain liquid under ambient temperature are still very limited [7]. 

The freezing point lowering (A 7 ) is determined from the molality (m) of the particles in the solution and the freezing point constant, Kf. 

Cryoscopy is the technique of precise determination of freezing points. As with ebulliometry, measurements of macromolecules with high molecular mass require the greatest accuracy in temperature difference measurements. As an example, benzene as a solvent with 0.1% of a solute macromolecule of 10,000 molecular mass shows a freezing point lowering of only 0.5 mK. 

Determination of Activities from Freezing Point Lowering of Soiutions 181... 

Pure ethylene glycol freezes at —I2,7°C, Exact composition and temperature for eutectic point are unknown, since solutions in this region turn to viscous, glassy mass that makes it difficult to determine the true freezing point. For the concentrations lower than eutectic, ice forms on freezing, while on the concentrated, solid glycol separates from the solution. 

M depends not on the molecular sizes of the particles but on the number of particles. Measuring colligative properties such as boiling point elevation, freezing point depression, and vapor pressure lowering can determine the number of particles in a sample. 

The lowering of freezing point and the generation of osmotic pressure both depend on the total concentration of solute particles. Therefore, by using the colligative property to determine the amount of solute present, and knowing its mass, we can infer its molar mass. 

This depression of the freezing point occurs in just the same way as the lower melting point of an impure sample, as discussed previously. This determination of the depression of the freezing point is termed crysoscopy. 

Methods for the determination of Molecular weight based on colligative property are vapour-pressure lowering, boiling point elevation (ebulliometry), freezing-point depression (cryoscopy), and the Osmotic pressure (osmometry). 

The freezing point of a solution of a nonvolatile solute is always lower than the pure solvent and the boiling point is always higher. It is the number of solute particles that determines the amount of the lowering of the freezing point and raising of the boiling point. 

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