Solutes nonvolatile

It seems appropriate to assume the applicability of equation (A2.1.63) to sufficiently dilute solutions of nonvolatile solutes and, indeed, to electrolyte species. This assumption can be validated by other experimental methods (e.g. by electrochemical measurements) and by statistical mechanical theory. 

Preparing a Volatile Sample Gas chromatography can be used to separate analytes in complex matrices. Not every sample that can potentially be analyzed by GG, however, can be injected directly into the instrument. To move through the column, the sample s constituents must be volatile. Solutes of low volatility may be retained by the column and continue to elute during the analysis of subsequent samples. Nonvolatile solutes condense on the column, degrading the column s performance. 

When a solution of a nonvolatile solute is heated, it does not begin to boil until the temperature exceeds the boiling point of the solvent. The difference in temperature is called the boiling point elevation, ATb. 

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. 

Boiling point elevation (ATb) Increase in the boiling point caused by addition of a nonvolatile solute, 269-271 Bomb calorimeter Device used to measure heat flow, in which a reaction is carried out within a sealed metal container, 202-203... 

The activity of a volatile solvent in a solution that contains a nonvolatile solute can be obtained from an experimental technique known as the isopiestic method .19 An apparatus is constructed similar to that shown in Figure 6.17. The mixture in container A is a solution of a nonvolatile solute in a solvent in which A], the activity of the solvent, has been accurately determined in other experiments as a function of concentration. Containers B and C hold solutions of other nonvolatile solutes in the same solvent. These are the solutions for which the activity of the solvent is to be determined. 

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. 

Organic chemists once used freezing-point and boiling-point measurements to determine the molar masses of compounds that they had synthesized. When 0.30 g of a nonvolatile solute is dissolved in 30.0 g of CC14, the boiling point of the solution is 77.19°C. What is the molar mass of the compound ... 

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. 

Sheehan, E. W., Ketkar, S. and Willoughby, R. C., Volatility enhancement of nonvolatile solutes by the combination of a heated target and a solvent depleted particle beam , in Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 19-24, 1991, pp. 1306-1307. 

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 effect of a solute on the boiling point of a solution is opposite to its effect on the freezing point. A nonvolatile solute inereases the boiling point of a solution. This is because the solute blocks some of the solvent molecules from reaching the surface of the solution and thus decreases the rate of escape into the gas phase. To get back to dynamic equilibrium, the solution must be heated so that more molecules acquire sufficient energy to escape from the liquid phase. 

The phase diagram for water, showing how the phase boundaries change when a nonvolatile solute dissolves in the liquid. 

When a nonvolatile solute is incorporated in a volatile solvent droplet, the dry particle diameter can be derived from the following expression ... 

The depression of the freezing point of a solvent due to the presence of a dissolved solute is an example of a colligative property, that is, a property of a dilute solution that depends on the number of dissolved particles and not on the identity of the particles. Water has a freezing point depression constant, Kf, of 1.86 K kg mol-1. In other words, for every mole of nonvolatile solute dissolved in a kilogram of water, the freezing point of water is lowered by 1.86°C. The change in freezing point, A T, can be calculated from the equation... 

The curves in Fig. 10 were drawn for the particular instance of a volatile solute dissolved in a volatile solvent, such as would exist for the acetone-chloroform system (whose diagram is very nearly like that of Fig. 10B). For many nonvolatile solutes, it not possible to trace smooth partial pressure curves across the entire range of mole fractions. This is especially true for aqueous salt solutions, where at a certain concentration of solute the solution becomes saturated. Any further addition of crystalline solute to the system does not change the mole fraction in the liquid phase, and the partial pressure of water thereafter remains constant, in accord with the phase rule. This phenomenon permits the use of saturated salt solutions as media to establish fixed relative humidity values in closed systems [12],... 

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. 

Raoult s law and colligative properties (nonvolatile solutes) osmosis... 

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 ... 

Solution atomization involves dissolution of a relatively nonvolatile liquid (solute) in a volatile solvent and atomization of the solution. During the atomization, the solvent material will evaporate in surrounding medium (air), leaving only nucleus droplets of the nonvolatile solute. The final droplet size is a function of the initial droplet size, the mass concentration of the solute, and the density ratio of the solution to the solute. The limitation of this technique lies in that it requires the dissolution of the liquid to be dispersed in a solvent. 

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. 

The freezing point of a solution of a nonvolatile solute is always lower than the pure solvent and the boiling point is always higher. It is the number of solute particles that determines the amount of the lowering of the freezing point and raising of the boiling point. 

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. 

By a similar set of arguments, it can be demonstrated that the boiling point elevation for dilute solutions containing a nonvolatile solute is given by the expression... 

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. 

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... 

Vapor Pressure Vapor pressure of solution with nonvolatile solute is lower than that of solvent... 

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