Water vapor pressure over salt solutions

In all such methods, the solvent activity is measured over a range of solute concentrations. For example, solvent activity for a vapor-saturated aqueous salt solution can be easily determined from the decrease in vapor pressure of the salt solution relative to that of pure water, using the apparatus illustrated in Figure 17.7a. This technique... 

For dilute solutions, the relationship between partial pressure and composition can be determined from theory over wider concentration range.s, it is in general necessary to determine the relationship by experiment. This has been done for solutions of certain salts and acids data for the equilibrium vapor pressure of water over solutions of sulfuric acid at 25°C are shown in Table 9.2. As the concentration of sulfuric acid increases, the vapor pressure of water over the solution drops sharply. 

Equilibrating membranes over salt solutions that set the vapor pressure have provided membrane water activity versus water content data In... 

The correct answer is (B). As the salt solution sits out, water will continue to evaporate while the amount of solute will remain constant. As a result, the concentration of the solution continues to increase over time. This causes the vapor pressure to decrease over time. 

The desiccator lid was manufactured with a ring stopcock through which evacuation was accomplished. A center tube was sealed on the lid, connecting it to a bulb. The bulb contained the saturated salt solution that maintained a constant water vapor pressure over the specimens. The bulb was isolable by means of a stopcock, so that the saturated salt solution could be stored under vacuum while samples were being changed. A ground joint connection made it possible to remove the whole upper assembly from the lid. Six such desiccators were employed, a different water vapor pressure being maintained in each. The relative... 

Let us consider a droplet of diameter Dp containing nw moles of water and ns moles of solute (e.g., a nonvolatile salt). If the solution were flat, the water vapor pressure over it would satisfy (17.14). Substituting this expression into (17.9), one obtains... 

Freezing-Point Depression Dissolution of a salt in water lowers the vapor pressure over the solution. A direct result of this is the depression of the freezing point of water. In order... 

Freezing Point Depression Dissolution of a salt in water lowers the vapor pressure over the solution. A direct result of this is the depression of the freezing point of water. In order to quantify this change, assume that our system has constant pressure and contains air and an aqueous salt solution in equilibrium with ice. According to thermodynamic equilibrium the chemical potential of water in the aqueous and ice phases will be the same, /a, = fiu,-If Ou, is the activity of water in solution then... 

One manometer has pure water over the mercury, one manometer has a 1.0 A/ solution of NaCl and the other manometer has a 1.0 A/ solution of urea. The pure water will have the highest vapor pressure and will thus force the mercury coluttm down the most coluum X. Both the salt and the urea will lower the overall pressure of the water. However, the salt dissociates into sodium and chloride ions (van t Hoff factor / = 2), whereas urea is a molecular compoimd with a van t Hoff factor of 1. Therefore the urea solution will lower the pressure only half as much as the salt solution. Y is the NaCl solution and Z is the urea solution. 

This initial stage of droplet formation deserves a careful explanation. Over a flat, pure water surface at 100% relative humidity (saturation with respect to water), water vapor is in equilibrium, which means that the number of water molecules leaving the water surface is balanced by the number arriving at the surface. Molecules at water surfaces are subjected to intermolecular attractive forces exerted by the nearby molecules below. If the water surface area is increased by adding curvature, molecules must be moved from the interior to the surface layer, in which case energy is required to oppose the cohesive forces of the liquid. As a consequence, for a pure water droplet to be at equilibrium, the relative humidity has to exceed the relative humidity at equilibrium over a flat, pure water surface, or be supersaturated. The flux of molecules to and from a surface produces what is known as vapor pressure. The equilibrium vapor pressure is less over a salt solution than it is over pure water at the same temperature. This effect balances to some extent the increase in equilibrium vapor pressure caused by the surface curvature of small droplets. Droplets with high concentrations of solute can then be at equilibrium at subsaturation. 

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