Solutions vapor pressures

Ammonia is readily absorbed ia water to make ammonia liquor. Figure 2 summarizes the vapor—Hquid equiUbria of aqueous ammonia solutions and Figure 3 shows the solution vapor pressures. Additional thermodynamic properties may be found ia the Hterature (1,2). Considerable heat is evolved duriag the solution of ammonia ia water approximately 2180 kJ (520 kcal) of heat is evolved upon the dissolution of 1 kg of ammonia gas. 

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

Now interpret phase X as pure solute then Cs and co become the equilibrium solubilities of the solute in solvents S and 0, respectively, and we can apply Eq. (8-58). Again the concentrations should be in the dilute range, but nonideality is not a great problem for nonelectrolytes. For volatile solutes vapor pressure measurements are suitable for this type of determination, and for electrolytes electrode potentials can be used. 

To evaluate the logarithm, we must measure the vapor pressure Pa of A in equilibrium with a solution where its mole fraction is XA in the limit where the solution becomes infinitely dilute. That is, in the limit of infinite dilution where y is 1, the free energy of solvation can be obtained from measurement of the solute vapor pressure (in the appropriate standard state units) over a solution of known concentration. 

In temperature programming, the temperature of a column is raised during the separation to increase solute vapor pressure and decrease retention times of late-eluting components. At a constant temperature of 150°C, the more volatile compounds in Figure 24-10 emerge close together, and less volatile compounds may not even be eluted from the column, [f the temperature is increased from 50° to 250°C at a rate of 8°/min, all compounds are eluted and the separation of peaks is fairly uniform. Do not raise the temperature so high that analytes and stationary phase decompose. 

For liquids with low water miscibility, AWPCs can be calculated as the ratio of the solute vapor pressure to the water solubility ... 

If an ionic substance such as NaCl is the solute, we have to calculate mole fractions based on the total concentration of solute particles (ions) rather than NaCl formula units. A solution of 1.00 mol NaCl in 15.0 mol water at 25°C, for example, contains 2.00 mol of dissolved particles (assuming complete dissociation), resulting in a mole fraction for water of 0.882 and a solution vapor pressure of 21.0 mm Hg. 

Show that the mole fraction of B in the vapor phase above an ideal binary solution consisting of components A and B with vapor pressures PJ and Pjf is yB = PB(P — P )/P(Pb P%)> when the solution vapor pressure is P. 

There are four main colligative properties, or properties of a solvent that are affected by the presence of a solute vapor-pressure reduction, boiling-point elevation, freezing-point depression, and osmotic pressure. 

Figure 9. Harness plot of binary Nd(NOs)s-HNOs-HgO electrolyte solution vapor pressure data as a function of ionic strength fraction of Nd(NOs)s...

Since we know how the solution vapor pressure varies with concentration (the relationship being given by Equation 6.5-2) and temperature (through the Clausius-Clapeyron equation. Equation 6.1-3), we can determine the relationships between concentration and both boiling point elevation and freezing point depression. The relationships are particularly simple for dilute solutions x — 0, where x is solute mole fraction). 

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. 

A trial calculation illustrates the nature of the solution. Vapor pressures are taken from Example 3a or 3fc a trial value of %i then allows calculation of Yi and Ys by Eqs. B) and (C) of Example 3. The values of Xi, K, and P that result are substituted into the summation equation. In the unlikely event that the sum is indeed unity, the chosen value of %i is correct. If not, then successive trials easily lead to this value. Note that the trivial solution giving E= 1 must be avoided. More elegant solution procedures can of course be employed. The answers are... 

Some properties in chemistry depend solely on the number of particles, irrespective of the type of particle. Such properties are called colligative. There are four colligative properties of solutions vapor pressure, boiling point, freezing point, and osmotic pressure. 

Increases in pressure increase the solubility of gaseous solutes, but have little effect on solid solutes. Similarly, decreases in pressure decrease the solubility of gases in liquids and have little effect on solid solutes. There are four main coUigative properties, or properties of a solvent that are affected by the presence of a solute vapor-pressure reduction, boiling-point elevation, freezing-point depression, and osmotic pressure. 

For a binary solution at constant composition, an expression of the form (9.1) is found for the slope of the vapor pressure curve as a function of temperature in which the latent heat of vaporization is the value for the solution. Solution vapor pressure curves can be represented as a set of parametric curves at constant composition on the vapor pressure diagram. 

Raoult s law states that in dilute solutions of nonvolatile nonelectrolytes the depression is proportional to the mole fracture of the solute, or the solution vapor pressure is proportional to the mole fraction of the solvent. In equation form ... 

Solution vapor pressure has a direct bearing on fractional distillation, a procedure for separating liquid components of a solution based on their different boiling points. Fractional distillation is somewhat analogons to fractional crystallization. Suppose we want to separate a binary system (a system with two components), say, benzene-toluene. Both benzene and toluene are relatively volatile, yet their boiling points are apprecia-... 

AP = X2P1 (12.4) pressure of a liquid to its vapor pressure in a solution. Vapor pressure lowering in terms of the... 

Pvap = "7 (pure solvent vapor pressure) + — (pure solute vapor pressure)... 

---