Vapor pressure with volatile solutes

Calculations of vapor pressure when volatile solutes are added can also be done but are more complicated since a volatile solute contributes to the vapor pressure along with the solvent. 

Recall from Section 10.5 that a liquid in a closed container is in equilibrium with its vapor and that the amount of pressure exerted by the vapor is called the vapor pressure. When you compare the vapor pressure of a pure solvent with that of a solution at the same temperature, however, you find that the two values are different. If the solute is nonvolatile and has no appreciable vapor pressure of its own, as occurs when a solid is dissolved, then the vapor pressure of the solution is always lower than that of the pure solvent. If the solute is volatile and has a significant vapor pressure of its own, as often occurs in a mixture of two liquids, then the vapor pressure of the mixture is intermediate between the vapor pressures of the two pure liquids. 

If the solute is a volatile solute (a solute with a vapor pressure), the situation is a little more complicated. A volatile solute will also compete for the surface area of a liquid. However, some of the molecules of a volatile solute will escape from solution and contribute to the vapor pressure. If the solution is an ideal solution (solute and solvent have similar properties), the partial pressures contributed by the solvent and solute can be found by applying Raoult s law separately. The sum of the partial pressures gives the total vapor pressure of the solution, and we arrive at a modified form of Raoult s law ... 

In a binary two-pbase system in which a solution of volatile solvent A and nonvolatile solute B is in equilibrium with gaseous A, the vapor pressure of the solution is equal to the system pressure p. 

Gas or vapor removal is more complex than particulate removal because the constituent removed from the air stream is dissolved in the liquid phase and may change the liquid properties. A solution of any volatile material exhibits a vapor pressure of the solute above the liquid phase. This vapor pressure increases with solute concentration and liquid temperature. Contaminant removal continues only as long as the partial pressure of that constituent in the gas phase exceeds the vapor pres-... 

Volatilizational loss of chemicals from water to air is an important fate process for chemicals with low aqueous solubility and low polarity. Many chemicals, despite their low vapor pressure, can volatilize rapidly owing to their high activity coefficients in solution. Volatilizational loss from surfaces is a significant transport process. Volatilization of organic chemicals from the soil surface is complicated by other variables. There is no simple laboratory... 

The vapor pressure of a polymer is, of course, far too small to measure We may, nevertheless, insist that such a vapor pressure exists, however small it may be. Or we may resort to the use of the escaping tendency, or fugacity, in place of the partial vapor pressure in the development given above, in accordance with usual thermodynamic procedures applied to the treatment of solutions. The treatment given here is in no way restricted to volatile solutes. 

Volatilization of contaminants from subsurface aqueous solutions into the subsurface gas phase or the (above ground) atmosphere is controlled by the vapor pressnre. Componnds with high vapor pressure tend to accnmnlate in the gas phase, which may be considered a kind of componnd solnbility in the atmosphere. Partitioning between the liquid and the gas phases is described by Henry s law and is expressed as... 

A. Vacuum Line Filtration. For their work on the diammoniate of dibo-rane. Parry, Schultz, and Girardot24 devised a versatile vacuum line filtration apparatus which is useful when small quantities of solid are handled and when the solvent is sufficiently volatile to be distilled on the vacuum line. The filter is attached to the vacuum system through a standard taper joint which allows it to be rocked or inverted (Fig. 9.21). Prior to filtration, any volatile contents are frozen down and the apparatus is thoroughly evacuated (Fig. 9.21a). By inversion of the apparatus, the solution is then poured onto the frit, and the solvent vapor pressure is employed to effect a suction filtration by closing the stopcock in the equalizing arm and cooling the lower tube (Fig. 9.21b). The precipitate is washed by distillation of the solvent from the lower receiver into the upper portion of the apparatus (with the stopcock in the sidearm open) and repetition... 

KEY CONCEPT PROBLEM 11.17 The following phase diagram shows part of the vapor-pressure curves for a pure liquid (green curve) and a solution of the first liquid with a second volatile liquid (red curve). 

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