Vapor pressure lowering and

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. 

Because the presence of a nonvolatile solute lowers the vapor pressure of the solvent, the boiling point of the solvent rises. This increase is called boiling-point elevation. The elevation of the boiling point has the same origin as vapor-pressure lowering and is also due to the effect of the solute on the entropy of the solvent. 

KELVIN EFFECT, VAPOR PRESSURE LOWERING, AND THE KOHLER CURVES... 

This relationship constitutes the basic definition of the activity. If the solution behaves ideally, a, =x, and Equation (18) define Raoult s law. Those four solution properties that we know as the colligative properties are all based on Equation (12) in each, solvent in solution is in equilibrium with pure solvent in another phase and has the same chemical potential in both phases. This can be solvent vapor in equilibrium with solvent in solution (as in vapor pressure lowering and boiling point elevation) or solvent in solution in equilibrium with pure, solid solvent (as in freezing point depression). Equation (12) also applies to osmotic equilibrium as shown in Figure 3.2. 

On examination of Equations 14 and 16, it is clear that estimates of vapor pressure lowering and ks are all that are required to determine the influence of a given salt on the vapor-liquid equilibrium behavior of a binary solvent mixture. 

SIDEBAR 7.12 VAPOR-PRESSURE LOWERING AND ASSOCIATED BOILING AND FREEZING POINT DISPLACEMENTS... 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

Suppo.se that a colligative property of the polymer solution is measured. These are properties that depend on the number of dissolved solute molecules and not on their sizes (see also Section 2.10). Osmotic pressure, vapor pressure lowering, and freezing point depression are some examples of colligative properties. If the value of the property measured is P, then by definition... 

The colligative properties we will be observing are freezing-point depression, boiling-point elevation, vapor-pressure lowering, and osmosis. Put on your safety glasses for all four demonstrations. 

The value of AT], is proportional to the vapor-pressure lowering, and so it is also proportional to the concentration (molality) of the solution. That is,... 

In addition to molarity and normality, another useful concentration unit is molality, m. A one-molal solution contains one mole per 1000 g of solvent. The molal concentration is convenient in physicochemical measurements of the colligative properties of substances, such as freezing point depression, vapor pressure lowering, and osmotic pressure because colligative properties depend solely on the number of solute particles present m solution per mole of solvent. Molal concentrations are not temperature dependent as molar and normal concentrations are (since the solvent volume in molar and normal concentrations is temperature dependent). 

In addition to freezing-point lowering and boiling-point raising, vapor-pressure lowering and osmotic pressure are also colligative properties. As we examine each one, notice how solute concentration quantitatively affects the property. 

Ebulliometry n. Method of measuring molecular weight of polymers under 20,000 based on vapor pressure lowering and boding point elevation. Pethrick RA, Dawkins JW (eds) (1999) Modern techniques for polymer characterization. John Wiley and Sons, New York. 

Raoult s law n. The quantitative relationship between vapor-pressure lowering and concentration in an ideal solution is stated in Raoult s law the partial vapor pressure of a component in solution is equal to the mole fraction of that component times its vapor pressure when pure at a temperature ... 

The binary sodium chloride-water system has been the object of many studies. As a result there is a wealth of published data for a wide range of temperatures. This data includes solubility, density, vapor pressure lowering and heat of solution measurements. Because of this availability of data and the straightforward strong electrolyte behavior of the system, sodium chloride has almost always been included as an example when illustrating activity coefficient modeling techniques. For this application, Meissner s method of activity coefficient calculation will be used. 

Note that X is smaller than 1, causing the positive entropy and negative contribution to the chemical potential stabilizing the solution. The vapor pressure lowering and other colligative properties are used to determine molecular masses of the solute 2. 

The second group of methods makes use of the colligative properties of solutions. Colligative properties depend on the number of molecules in a solution, and not their chemical constitution (22). The colligative properties include boihng point elevation, melting point depression, vapor pressure lowering, and osmotic pressure. The basic equations for the first two may be written (23)... 

Calculating vapor-pressure lowering Given the mole fraction of solute in a solution of nonvolatile, undissociated solute and the vapor pressure of pure solvent, calculate the vapor-pressure lowering and vapor pressure of the solution. (EXAMPLE 12.9)... 

Colligative properties of solutions depend only on the total number of solute particles present. Boiling-point elevation, freezing-point depression, vapor-pressure lowering, and osmotic pressure are colligative properties. 

Include boiling point elevation, freezing point depression, vapor pressure lowering, and osmosis... 

Colligative properties are properties that depend on the concentration of a solute but not on its identity. The name comes from a Latin word meaning tied together and is used because of the common dependence that these properties have on solute concentfation. The four principal colligative properties are freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. 

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. 

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