Lowering of Vapor Pressure

A pure liquid A is assumed to be in equilibrium with its vapor at a pressure of pig (initial situation = PA g)- Th vapor pressure of A is lowered if a low-volatile 

Qualitatively, this can be understood immediately. Adding B dilutes liquid A l, thereby lowering its chemical potentialp ii- Th dissolved substance B should have low volatility so it contributes nothing to the vapor A g and the potential remains unchanged. Because //a i i now lower than pA g th vapor has to condense on the surface of the solution, thereby causing the pressure to fall. 

We act on the assumption that /J n = therefore these contributions cancel each other out. Because we also have A g = A g = RT/p (see Sect. 9.3), the equation can be simplified according to  

In this context, p corresponds to the vapor pressure pig of the pure solvent. If it is important to emphasize this fact, we add the symbol and write instead of pig. For the lowering of vapor pressure Apig, we obtain a relation that was discovered empirically in 1890 by the French Chemist Franfois Marie Raoult  

Let us have a look at Fig. 12.8 for illustration At the intersection of the potentials for pure solvent and pure vapor, there is equilibrium between the liquid and its vapor phase at the vapor pressure pig. A dissolved substance of low volatility lowers the chemical potential of the solvent by RTxb ( colligative potential lowering corresponding to the distance between the almost horizontal straight lines), but 

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Molecular Weight Determination by Application of Raoult s Law. If a small amount (m in grams) of a nonvolatile, nonionized substance (solute, 2) is dissolved in m, grams of a volatile liquid (solvent, 1), it experiences a lowering of vapor pressure from the pure solvent value (P ) to the solution value (P) at the system temperature. This is a consequence of Raoult s law because the total vapor pressure of the dilute solution (x 1) is given by P = x P + x P = 1 -... 

CH,CH(CN)—. (c) What would be the vapor pressure of the solution if the vapor pressure of pure water at 25°C is 0.0313 atm (Assume that the density of the solution is 1.00 g-ern .) (d) Which approach (osmometry or the lowering of vapor pressure) would you prefer for the determination of very high molar masses such as those of acrylic resins Why ... 

There are several basic physical-chemical principles involved in the ability of aerosol particles to act as CCN and hence lead to cloud formation. These are the Kelvin effect (increased vapor pressure over a curved surface) and the lowering of vapor pressure of a solvent by a nonvolatile solute (one of the colligative properties). In Box 14.2, we briefly review these and then apply them to the development of the well-known Kohler curves that determine which particles will grow into cloud droplets by condensation of water vapor and which will not. 

In the 1920s, it was not feasible to accurately measure the molecu-f lar weight of natural or synthetic polymers. Classical methods 1 of molecular weight determina-V tion, those based upon colligative x properties, elevation of boiling point, depression of freezing point and lowering of vapor pressure, worked very well for low-molar-mass compounds, but were essentially useless for macromolecules. Modern instrumental methods that... 

Lowering of Vapor Pressure by Salts in Aqueous Solutions... 

The lowering of vapor pressure depends on the number of solute particles present in the solution. 

Free energy of solution, AGsoi < 0, leading to a lowering of vapor pressure of the solvent (see Section 8.2). 

Babo s law. The lowering of vapor pressure is proportional to the mole fraction of nonvolatile solute in a solution. 

Carry out calculations involving the four colligative properties of solutions lowering of vapor pressure (Raoult s Law), boiling point elevation, freezing point depression and osmotic pressure... 

Figure 14-8 Lowering of vapor pressure. If no air is present in the apparatus, the pressure above each liquid is due to water vapor. This pressure is less over the solution of sugar and water.

The lowering of vapor pressure at any temperature then follows from Raoult s law,... 

A substance in solution has a chemical potential, which is the partial molar free energy of the substance, which determines its reactivity. At constant pressure and temperature, reactivity is given by the thermodynamic activity of the substance for a so-called ideal system, this equals the mole fraction. Most food systems are nonideal, and then activity equals mole fraction times an activity coefficient, which may markedly deviate from unity. In many dilute solutions, the solute behaves as if the system were ideal. For such ideally dilute systems, simple relations exist for the solubility of substances, partitioning over phases, and the so-called colligative properties (lowering of vapor pressure, boiling point elevation, freezing point depression, osmotic pressure). 

Note Equation (8.2) often is called Raoult s law. Strictly speaking, however, Raoult s law only applies to lowering of vapor pressure by a solute. Cf. Eq. (8.6). 

An illustration of Raoult s law lowering of vapor pressure by addition of solute molecules. White units represent solvent molecules, and red units are solute molecules. Solute molecules present a barrier to escape of solvent molecules, thus decreasing the vapor pressure. 

LOWERING OF VAPOR PRESSURE BY SALTS IN AQUEOUS SOLUTIONS... 

There are several disadvantages with this method and it is therefore used largely for demonstration purposes. The main problem is that the exact amount of solvent remaining in the liquid phase is unknown and varies with the rate of boiling. The theoretical explanation of the effect is identical to that for the lowering of vapor pressure the boiling points are those tem-... 

Osmosis is a colligative property and its theoretical treatment is similar to that for the lowering of vapor pressure. The membrane can be regarded as equivalent to the liquid-vapour interface, i.e. one that permits free movement of solvent molecules but restricts the movement of solute molecules. The solute molecules occupy a certain area at the interface and therefore inhibit solvent egress from the solution. Just as the development of a vapor pressure in a closed system is necessary for liquid-vapour equilibrium, the development of an OSMOTIC pressure on the solution side is necessary for equilibrium at the membrane. 

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