Vapor pressures of solutions

Temperature, °C Urea ia solution, wt % Vapor pressure of solution, kPa... 

The isopiestic method was first introduced by W. R. Bousfield. "Isopiestic Solutions." Trans. Faraday Soc., 13, 401-410 (1918) and refined by D. A. Sinclair. A Simple Method for Accurate Determinations of Vapor Pressure of Solutions". J. Plus. Chem., 37.495-504 (1933). 

Roughly half of the data on the activities of electrolytes in aqueous solutions and most of the data for nonelectrolytes, have been obtained by isopiestic technique. It has two main disadvantages. A great deal of skill and time is needed to obtain reliable data in this way. It is impractical to measure vapor pressures of solutions much below one molal by the isopiestic technique because of the length of time required to reach equilibrium. This is generally sufficient to permit the calculation of activity coefficients of nonelectrolytes, but the calculation for electrolytes requires data at lower concentrations, which must be obtained by other means. 

His works are known in studies on osmotic pressures and vapor pressures of solutions, but he is much better known as the discoverer of glutamic acid extracted from sea tangle (Konbu, a seaweed, in Japanese) and the producer of the seasoning Aji-no-Moto (sodium glutamate). His wife, Sada, is a younger sister of the wife of Joji Sakurai. 

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. 

Vapor Pressure Vapor pressure of solution with nonvolatile solute is lower than that of solvent... 

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. 

AP = (vapor pressure of pure solvent) - (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. 

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

Remember that a saturated solution has the same vapor pressure of solute as the crystalline solute in equilibrium with it. 

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. 

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