Vapor pressure nonvolatile solute

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. 

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 ... 

Molecular Weight Determination by Application of Raoult s Law. If a small amount (m in grams) of a nonvolatile, nonionized substance (solute, 2) is dissolved in m, grams of a volatile liquid (solvent, 1), it experiences a lowering of vapor pressure from the pure solvent value (P ) to the solution value (P) at the system temperature. This is a consequence of Raoult s law because the total vapor pressure of the dilute solution (x 1) is given by P = x P + x P = 1 -... 

Boiling point elevation is a direct result of vapor pressure lowering. At any given temperature, a solution of a nonvolatile solute has a vapor pressure lower than that of the pure solvent. Hence a higher temperature must be reached before the solution boils, that is, before its vapor pressure becomes equal to the external pressure. Figure 10.8 (p. 270) illustrates this reasoning graphically. 

Effects of vapor pressure lowering. Because a nonvolatile solute lowers the vepor pressure of a solvent, the boiling point of a solution will be higher and the freezing point lower than the corresponding values for the pure solvent Water solutions freeze below 0°C at point A and boil above 100°C at point B. 

Calculate the vapor pressure of water over each of the following ethylene glycol (C2H602) solutions at 22°C (vp pure water = 19.83 mm Hg). Ethylene glycol can be assumed to be nonvolatile. 

FIGURE 8.28 The vapor pressure of a solvent is lowered by a nonvolatile solute. The barometer tube on the left has a small volume of pure water floating on the mercury. That on the right has a small volume of 10 m NaCI(aq) and a lower vapor pressure. Note that the column on the right is depressed less by the vapor in the space above the mercury than the one on the left, showing that the vapor pressure is lower when solute is present. 

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. 

Benzene has a vapor pressure of 94.6 Torr at 25°C. A nonvolatile compound was added to 0.300 mol benzene at 25°C and the vapor pressure of the benzene in the solution decreased to 75.0 Torr. What amount (in moles) of solute molecules was added to the benzene ... 

Raoult s law The vapor pressure of a liquid solution of a nonvolatile solute is directly proportional to the mole fraction of the solvent in the solution P = soiventPm,re, where Ppurc is the vapor pressure of the pure solvent. 

Equation describes the total vapor pressure above a solution when the solute does not have a significant vapor pressure of its own. In other words, Raoulfs law applies only to nonvolatile solutes. When the solute is volatile, such as for a solution of acetone in water, the total vapor pressure above the solution is a sum of contributions from both solvent and solute. 

The vapor pressure of pure water under these conditions is 23.76 torr. Any nonvolatile solute reduces the vapor pressure, so we expect a result that is smaller than this value. 

The vapor pressure of a liquid depends upon the ease with which the molecules are able to escape from the surface of the liquid. The vapor pressure of a liquid always decreases when nonvolatile solutes (ions or molecules) are dissolved in it, since after dissolution there are fewer solvent molecules at the surface to vaporize. 

The vapor pressure of an ideal solution is 450. mm Hg. If the vapor pressure of the pure solvent is 1000. mm Hg, what is the mole fraction of the nonvolatile solute ... 

Background If a nonvolatile solid is dissolved in a liquid, the vapor pressure of the liquid solvent is lowered and can be determined through the use of Raoult s Law, Pi = X 0. Raoult s Law is valid for ideal solutions wherein AH = 0 and in which there is no chemical interaction among the components of the dilute solution (see Figure 1). 

If a pure liquid is the solvent and you add a nonvolatile solute, the vapor pressure of the resulting solution is always less than the pure liquid. The addition of the solute lowers the vapor pressure and the amount of lowering is proportional to the number of solute particles added. 

If a liquid is placed in a sealed container, molecules will evaporate from the surface of the liquid and eventually establish a gas phase over the liquid that is in equilibrium with the liquid phase. The pressure generated by this gas is the vapor pressure of the liquid. Vapor pressure is temperature-dependent the higher the temperature, the higher the vapor pressure. If the liquid is made a solvent by adding a nonvolatile solute, the vapor pressure of the resulting solution is always less than that of the pure liquid. The vapor pressure has been lowered by the addition of the solute the amount of lowering is proportional to the number of solute particles added and is thus a colligative property. 

The isopiestic method has been used frequently to measure the vapor pressure of aqueous solutions of nonvolatile solutes. 

The activity of the solvent often can be obtained by an experimental technique known as the isopiestic method [5]. With this method we compare solutions of two different nonvolatile solutes for one of which, the reference solution, the activity of the solvent has been determined previously with high precision. If both solutions are placed in an evacuated container, solvent will evaporate from the solution with higher vapor pressure and condense into the solution with lower vapor pressure until equilibrium is attained. The solute concentration for each solution then is determined by analysis. Once the molality of the reference solution is known, the activity of the solvent in the reference solution can be read from records of previous experiments with reference solutions. As the standard state of the solvent is the same for all solutes, the activity of the solvent is the same in both solutions at equUibrium. Once the activity of the solvent is known as a function of m2 for the new solution, the activity of the new solute can be calculated by the methods discussed previously in this section. 

Fig. 1, the metal atoms may be generated in an electrically heated crucible and co-condensed with the substrate on the cold walls of the reaction vessel. To minimize gas-phase reactions, a good vacuum must be maintained in the reactor during this codeposition. An alternative procedure is to condense the metal vapor into a well-stirred solution of the reactant in a suitable solvent cooled to a temperature at which the vapor pressure of the solution is <10 3 torr. This method has special advantages for the preparation of unstable organometallic compounds and for reacting metal atoms with nonvolatile substrates.2... 

To examine the four properties listed in Table 11.4, we can use the simple case of a nonelectrolyte, nonvolatile solute. The lowering of the vapor pressure is a consequence of nonvolatile solute particles occupying positions at the surface of the... 

There are several basic physical-chemical principles involved in the ability of aerosol particles to act as CCN and hence lead to cloud formation. These are the Kelvin effect (increased vapor pressure over a curved surface) and the lowering of vapor pressure of a solvent by a nonvolatile solute (one of the colligative properties). In Box 14.2, we briefly review these and then apply them to the development of the well-known Kohler curves that determine which particles will grow into cloud droplets by condensation of water vapor and which will not. 

Thus, if a nonvolatile solute is dissolved in water, the vapor pressure of water is lowered by an amount proportional to the mole fraction of dissolved solute, taking into account any dissociation that occurs (vide infra). It should be noted that this assumes ideal solution behavior. 

For the limiting case of a dilute Henry s law solution of a nonvolatile solute (e.g., salt or other soluble solid) with PA 0, the expression for the vapor pressure of the solution reduces to the simplified form... 

It has long been known that salt lakes evaporate more slowly than freshwater lakes. The slower evaporation of salt lakes implies that the presence of the salt has reduced the vapor pressure. We see the same phenomenon in the laboratory. The vapor pressure of a solvent is found to be lower when a nonvolatile solute is present (Fig. 8.30). 

To understand why a solute lowers the vapor pressure, we need to look at the thermodynamic properties of the solution. We saw in Section 8.2, specifically Eq. 1, that at equilibrium, and in the absence of any solute, the molar free energy of the vapor is equal to that of the pure solvent. We now need to consider the molar free energies of the solvent and the vapor when a solute is present. We shall consider only nonvolatile solutes, which do not appear in the vapor phase, and limit our considerations to ideal solutions. 

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. 

It s easy to demonstrate with manometers that a solution of a nonvolatile solute has a lower vapor pressure than the pure solvent (Figure 11.9). Alternatively, you can show the same effect by comparing the evaporation rate of pure solvent with the evaporation rate of a solution. A solution always evaporates more slowly than a pure solvent does, because its vapor pressure is lower and its molecules therefore escape less readily. 

FIGURE 11.9 The equilibrium vapor pressure of a solution with (a) a nonvolatile solute is always lower than that of (b) the pure solvent by an amount that depends on the mole fraction of the solvent. 

Z58 For a solution of a nonvolatile / V I solid in a liquid, the vapor pressure of a solution is always lower than the vapor pressure of the pure solvent. For a solution of two volatile liquids, the vapor pressure is lower than the vapor pressure of the more volatile liquid and higher than the vapor pressure of the less volatile liquid. 

We saw in Section 10.5 that the vapor pressure of a liquid rises with increasing temperature and that the liquid boils when its vapor pressure equals atmospheric pressure. Because a solution of a nonvolatile solute has a lower vapor pressure than a pure solvent has at a given temperature, the solution must be heated to a higher temperature to cause it to boil. Furthermore, the lower vapor pressure of the solution means that the liquid /vapor phase transition line on a phase diagram is always lower for the solution than for the pure solvent. As a result, the triplepoint temperature Tt is lower for the solution, the solid/liquid phase transition line is shifted to a lower temperature for the solution, and the solution must be cooled to a lower temperature to freeze. Figure 11.12 shows the situation. 

What is the vapor pressure (in mm Hg) of the following solutions, each of which contains a nonvolatile solute The vapor pressure of water at 45.0°C is 71.93 mm Hg. 

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