Aqueous solutions vapor-pressure lowering

NaCl molecules in its aqueous solution. Vapor pressure lowering of dilute NaCl solutions obeys Raoult s law for the solvent water—that is, pw = PwXw— and the vapor pressure corresponds to fwo solute species. 

E7.14 Estimate the vapor pressure lowering and the osmotic pressure at 293.15 K for an aqueous solution containing 50.0 g of sucrose (Mi = 0.3423 kg-mol"1) in 1 kg of water. At this temperature, the density of pure water is 0.99729 g em"3 and the vapor pressure is 2.33474 kPa. Compare your results with those given in Table 7.3. 

Lindsay, W. T., Jr. Liu, C. T. "Vapor Pressure Lowering of Aqueous Solutions at Elevated Temperatures" Office of Saline Water, U.S. Government Printing Office,... 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

Calculate the vapor-pressure lowering in a solution of 2.00 m aqueous sucrose (table sugar) at 25°C. The vapor pressure of pure water at that temperature is 3.20 kPa. 

From this equation, you can see that the vapor-pressure lowering is a colligative property—one that depends on the concentration, but not on the nature, of the solute. Thus, if the mole fraction of ethylene glycol, X, in an aqueous solution is doubled from 0.010 to 0.020, the vapor-pressure lowering is doubled from 0.18 nunHg to 0.36 mmHg. Also,... 

In Chapter 14 (Solutions and Their Physical Properties), we have added a section to describe the standard thermodynamic properties of aqueous ions. We use the concepts of entropy and chemical potential in Chapter 13 to explain vapor pressure lowering and why gasoline and water don t mix. 

Extensive hydrogen bonding takes place in phosphoric acid solutions. In concentrated (86% H PO solutions, as well as in the crystal stmctures of the anhydrous acid and the hemihydrate, the tetrahedral H PO groups are linked by hydrogen bonding. At lower (75% H PO concentrations, the tetrahedra are hydrogen-bonded to the water lattice. Physical properties of phosphoric acid solutions of various concentrations are Hsted in Table 2 the vapor pressure of aqueous H PO solutions at various temperatures is given in Table 3. 

Dilute (1—3%), chloride-containing solutions of either HOCl, hypochlorite, or aqueous base, can be stripped in a column against a current of CI2, steam, and air at 95—100°C and the vapors condensed giving virtually chloride-free HOCl solutions of higher concentration in yields as high as 90% (122—124). Distillation of more concentrated solutions requires reduced pressure, lower temperature, and shorter residence times to offset the increased decomposition rates. 

The freezing points of electrolyte solutions, like their vapor pressures, are lower than those of nonelectrolytes at the same concentration. Sodium chloride and calcium chloride are used to lower the melting point of ice on highways their aqueous solutions can have freezing points as low as —21 and — 55°C, respectively. 

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. 

This reaction is highly exothermic. If the heat of the reaction is not conducted thru the walls of a closed container at a rate capable of maintaining an equilibrium temperature, an increase in pressure results with an increase in reaction rate, leading to explosive conditions. Acid salts, such as stannic chloride and zinc chloride, and bases, such as alkali metal hydroxides, either solid or in aqueous solution, and tertiary amines are all effective catalysts. It is, therefore, imperative that the concentration of such contaminants be kept at a minimum when transporting or storing sizeable quantities of ethylene oxide Accdg to Hess Tilton (Ref 16), a 90% decompn takes place if 100% vapor of EtnO in a closed container is. initiated with MF. There is no upper limit of EtnO in air (the previously reported value of 80% was in error), but the lower expl limit is 3% (Ref 17, p 87)... 

Deliquescence.—It sometimes happens that a salt that has been crystallized and spread in the air to dry absorbs water instead of losing it and in time passes again into solution. This may occur with anhydrous salts as well as with hydrates but not so frequently. This conduct is readily understood if it is remembered that the process of drying is always dependent upon the equilibrium between the aqueous vapor pressure of the material undergoing drying and the partial pressure of the water vapor in the air. The vapor pressure of a salt solution is always lower than that of the pure solvent. When the solution is very concentrated, its vapor pres-... 

Liquid and vapor are at equilibrium along the vapor pressure curves shown for pure water (solid line) and an aqueous solution (dashed line). The vapor pressure is lower for the solution, in accord with Raoult s law, and thus the boiling point is increased (liquids boil at 1 atm)... 

In our laboratories, extensive use has been made of vapor pressure (14-18) and membrane methods ( 2, 3, 19, 20) to Infer thermodynamic results for ternary aqueous systems containing an ionic or a nonionic surfactant and an organic solute. The most precise solubilization measurements ever reported have been obtained with an automated vapor pressure apparatus for volatile hydrocarbon solutes such as cyclohexane and benzene, dissolved In aqueous solutions of sodium octylsulfate and other Ionic surfactants (15, 16). A manual vapor pressure apparatus has been employed to obtain somewhat less precise results for solutes of lower volatility (17, 18). Recently, semi-equilibrium dialysis (19, 20) and MEUF (2) methods have been used to investigate solute-surfactant systems in which the organic solubilizates are too involatile to study by ordinary vapor pressure methods. 

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

Osmotic membrane distillation is one of the membrane distillation variants that operate at ambient temperature and atmospheric pressure. In OMD, a microporous hydrophobic membrane separates the two aqueous solutions, namely, feed and osmotic agent (OA) having different solute concentrations (osmotic pressure). The driving force in OMD is vapor pressure difference across the membrane. Water evaporates from the surface of the feed solution having higher vapor pressure, diffuses through the membrane in the form of vapor, and condenses on the surface of a solution with a lower vapor pressure. This results in the concentration of feed and dilution of osmotic agent solution. 

Example 2.3. Activity and Activity Coefficient of Aqueous NaCl Water vapor pressures have been measured over NaCl solutions of varying molal concentrations. Results of such measurements allow calculations of relative vairor pressure lowering, 4>, and 7., for a range of NaCl concentrations in water. Robinson and Stokes (1959) provide such data for concentrations ranging fn)m 0.1 to 6.0 molal. Table 2.1 shows results for three different concentrations. The mole fraction of H2O, Xfy, is also included. From such data the activity of aqueous NaCl can be computed, (see also Figure 2.4.)... 

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