The Vapor Pressures of Solutions

Liquid solutions have physical properties significantly different from those of the pure solvent, a fact that has great practical importance. For example, we add antifreeze to the water in a car s cooling system to prevent freezing in winter and boiling in summer. We also melt ice on sidewalks and streets by spreading salt. These preventive measures work because of the solute s effect on the solvent s properties. 

We can account for this behavior in terms of the simple model shown in Fig. 17.9. The dissolved nonvolatile solute decreases the number of solvent molecules per unit volume. Thus it lowers the number of solvent molecules at the surface, which proportionately lowers the escaping tendency of the solvent molecules. For example, in a solution consisting of half nonvolatile solute molecules and half solvent molecules, we expect the observed vapor pressure to be half that of the pure solvent, since only half as many molecules can escape. In fact, this agrees with our observations. 

Detailed studies of the vapor pressures of solutions containing nonvolatile solutes were carried out by Franqois M. Raoult. His results are described by the equation known as Raoult s law  

The presence of a nonvolatile solute inhibits the escape of solvent molecules from the liquid and so lowers the vapor pressure of the solvent. 

Raoult s law states that the vapor pressure of a solution is directly proportional to the mole fraction of solvent present 

To explore how a nonvolatile solute affects a solvent, we will consider the experiment represented in Fig. 17.8, in which a sealed container encloses a beaker containing an aqueous sugar solution and a beaker containing pure water. Gradually, the volume of the sugar solution increases and the volume of the pure water decreases. Why We can explain this observation if the vapor pressure of the pure solvent is greater than that of the solution. Under these conditions, the pressure of vapor necessary to achieve equilibrium with the pure solvent is greater than that required to reach equilibrium with the solution. Thus, as the pure solvent emits vapor in an attempt to reach equilibrium, 

An aqueous solution and pure water in a closed environment, (a) Initial stage, (b) After a period of time, the water is transferred to the solution. 

IBLG See questions from The Vapor Pressures of Solutions  

A nonvolatile solute has no tendency to escape from solution Into the vapor phase. 

Vapor Pressure Lowering Liquid/ Vapor Equilibrium 

Vapor Pressure Lowering Solution/ Vapor Equilibrium 

We can account for this behavior in terms of the simple model shown in Fig. 11.10. The dissolved nonvolatile solute decreases the number of solvent molecules per unit 

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This is a form of Henry s law, where the Henry s law constant is simply the product of the vapor pressure of solute 2 multiplied by the energy correction term exp(Ae/ 7 ). For oleophilic substances, which do not like water, mixing is an endothermic process and the term Ae/kT is positive, so that Henry s law constant is much higher than the vapor pressure of the pure substance. 

According to (7.65), the vapor-pressure lowering AP = PA — P is directly proportional to xB. As shown in the diagram, the dashed vapor-pressure curve for solution therefore falls below the solid pure-solvent curve, corresponding to the well-known effect of a nonvolatile solute in reducing the vapor pressure of solution. 

To calculate activities in the present scheme one determines the vapor pressures of solutions for various xif with special emphasis on the range where Henry s Law holds. Extrapolation of the straight line to xA — 1 yields Pf. Then XjPf corresponds to the vector DE and Pi, to the vector DF in Fig. 3.11.1. A measurement of PA corresponding to xA then yields aA and 7i. For more precise work, Eq. (3.11.6) must be employed. 

The vapor pressure of solutions in relation to mole fraction—Raoult s law. Freezing-point lowering and boiling-point rise. Osmotic pressure. Activity of ions. 

The effect of temperature on the solubility of PEG(400) In the SCF phase and CO2 in the polymer phase is shown in Figures 6 and 7 respectively. In the SCF phase (Figure 6), a temperature change of 10 dose not affect the solubility of PEG(400) in CO2. This observation suggests that the effects of the vapor pressure of solute and the density of the solvent are to some extent compensating. In the polymer phase (Figure 7), the solubility of CO2 drops with temperature because CO2 is very volatile and evaporates out of the liquid phase very effectively when temperature is increased from 313 to 323 K. 

C is correct. The vapor pressure of solution might be lower than just one of the pure substances but not the other. You can see this from the graph below. 

Using the distribution function (i.e., Eq. (10.3)), several average values can be defined. The number-average segment number, which is proportional to the number-average molar mass, which can be measured using the vapor pressure of solutions or osmometry, is given by the ratio of the 0 and -1 moment of the distribution function ... 

Margules (1895) suggested an equation for the vapor pressures of solution components (partial pressures) in the form of a series expansion, using the systems ethyl alchohol-water and methyl alchohol-water as examples. Using our notation, his equation was... 

The method of obtaining ex and n is briefly thus The vapor pressures of solutions were found (8,9g) to obey Raoult s (37) law, on correcting it for hydration (34,38) and incomplete dissociation (2,34) of the electrolyte. Thus, the vapor pressure ratio (data stored in the form of a and p in ( 3)) gives the mole fraction N, of free water. 

Besides methods which involve determination of phase compositions of equilibrium associations, other approaches to phase equilibria studies are possible. An example is the special method for determining the vapor pressure of solutions with a given composition ( Vap.pr. and Vap.pr.diff in Table 1.1). In such apparatus the composition is not measured but taken from the initial charge, whereas the vapor pressure is measured directly with a pressure gage (Mashovets et al, 1973 Bhatnagar and Campbell, 1982 ... 

Figure 13.11 Effect of solute on the vapor pressure of solution, A, Equilibrium between a liquid...

Separations in GLC are the resultant of selective solute-stationary-phase interactions and differences in the vapor pressure of solutes. The main forces that are responsible for solute interaction with a stationary phase are dispersion, induction, orientation and donor-acceptor interactions (25-27), the sum of which serves as a measure of the polarity of the stationary toward the solute. Selectivity, on the other hand, may be viewed in terms of the magnitude of the individual energies of interaction. In GLC, the selectivity of a column governs band spacing or the degree to which peak maxima are separated. The following parameters influence selectivity ... 

The key to linking the properties of a solution to those of a gas and setting up an expression for the chemical potential of a solute is the work done by the French chemist Fran( ois Raoult (1830-1901), who spent most of his fife mccisuring the vapor pressures of solutions. He measured the partial vapor pressure, pj, of each component in the mixture, the partial pressure of the vapor of each component in... 

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