Vapour pressure lowering

The change of vapour pressure following the addition of a nonvolatile solute to a solvent may be determined by application of Raoult s law. Since the solute is nonvolatile, the total vapour pressure, P, above the dilute solution is 

Reproduced from A. Cammarata, j, Pharm. Sci., 64, 2025 (1975) with permission. 

That is, the relative lowering of the vapour pressure is equal to the mole fraction of the solute. 

A direct consequence of the reduction of vapour pressure by the added solute is that the temperature at which the vapour pressure of the solution attains atmospheric pressure (that is, the boiling point) must be higher than that of the pure solvent. 

In Fig. 2.8, points A and B represent the boiling points of pure solvent and solution respectively. The boiling point is thus raised by an amount T-Tq = AT. A-E represents the lowering of vapour pressure, p -p, by the solution. 

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In physical chemistry, we apply the term colligative to those properties that depend upon number of molecules present. The principal colligative properties are boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. All such methods require extrapolation of experimental data back to infinite dilution. This arises due to the fact that the physical properties of any solute at a reasonable concentration in a solvent are... 

The technique is useful in that only small amounts of the sample polymer are needed, though experimentally it is time-consuming and may require great patience in use. This is because the technique does not measure equilibrium vapour-pressure lowering, but measures vapour-pressure lowering in a steady-state situation. Thus care must be taken to ensure that time of measurement and droplet size are standardised for both calibration and sample measurement. 

Colligative1 properties of dilute polymer solutions depend only on the number of dissolved molecules and not on properties of the molecules themselves, such as mass or size. Osmotic pressure, freezing point depression, boiling point elevation, and vapour pressure lowering are the most prominent examples. These methods essentially allow one to count the number n of solute molecules. From n and the known total mass m of the solute the molar mass M is readily obtained as... 

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). 

Of the osmotic methods I would mention only two that we have examined and used at Aberystwyth. The indirect determination of vapour pressure lowerings can be made simply, quickly, and precisely using thermistors an accuracy equivalent to one-half per cent in AP is readily attained for M/20 solutions in benzene this is equivalent to a... 

This expression, known as the Kelvin equation, has been verified experimentally. It can also be applied to a concave capillary meniscus in this case the curvature is negative and a vapour pressure lowering is predicted (see page 125). 

The negative deviation, on the other hand, is characterised by vapour pressures lower than those calculated for ideal solution. If the attraction between the unlike molecules (/-j) is stronger than the mutual attraction of the like molecules (i-i or j-j), then die escaping tendencies of the molecules in the solutions are lower than escaping tendencies in the individual liquids. Thus... 

The vapour pressure lowering (P — P) observed by the addition of B to pure A and caused by adding an involatile component B (mole fraction = xb) is given by ... 

It is known from physical chemistry that the equilibrium vapour pressure is smaller over solutions than over pure water. In the case of ideal solutions this vapour pressure decrease is proportional to x0, the mole fraction of the solvent (Raoult s law). If the solution is real, the interaction of solvent and solute molecules cannot be neglected. For this reason a correction factor has to be applied to calculate the vapour pressure lowering. This correction factor is the so-called osmotic coefficient of water (g ). We also have to take into account that the soluble substance dissociates into ions, forming an electrolyte. 

In the past, the basic experiment used to study the general properties of solutions was the measurement of the vapour pressure lowering of the solvent (Raoult 1890). But this method which is, in general, efficient is not really useful when the vapour pressure reduction is small. In such situations, it has been replaced by another method which is quite similar it consists in measuring the osmotic pressure of the solution with respect to the solvent. 

In the more concentrated case, the osmotic pressure was obtained by measurement of the solvent vapour-pressure-lowering (Chapter 5, Section 1.5). 

Boiling point elevation Freezing point depression Osmotic pressure Vapour pressure lowering... 

This is a widely used technique based on the determination of colligative properties. Despite its name, it is not ait osmotic technique at all, but is actually an indirect method of measuring vapour-pressure lowering. The parameter that is measured is the miniscule temperature difference that is obtained in an atmosphere of saturated solvent vapour between droplets of pure solvent and droplets of polymer solution each experiencing solvent evaporation and condensation. This small temperature difference is proportional to the vapour-pressure lowering of the polymer solution at equilibrium hence, it is also proportional to... 

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