Molal salt concentration

The (able below contains solubility and density data lor the salts Na SQ4 and MgS()4. Express their solubilities in terms of molar concentrations, molalities and mole fractions. Calculate the contractions in volume that occur when the solutions are made from the solid salts and the solvent. Comment on the results in terms of the ell ect of ionic charges. The concentrations have been cho-.cn to be comparable. 

Molal salt concentrations must be translated into mean ionic activity (Eq. (21.7)). The mean ionic activity coefficients of many salts are compiled in the literature as a function of salt concentration (see appendix 8.10... 

If a is the initial concentration (molality) of the weak or moderately weak acid HA, and h is the amount of strong, monoacid base MOH added at any instant, then h is also equal to mM > the molality of ions at that instant, since the salt MA produced on neutralization may be taken as being completely dissociated. The acid HA is only partially neutralized to form A ions, and so... 

One of the most difficult problems in working with natural aqueous solutions is assigning activities to trace or minor components that occur in concentrated salt solutions. An example might be calculating the activity coefficient of ppm ( lO m) concentrations of metal ions in hydrothermal solutions containing 1 to 5 molal concentrations of salts such as NaCl, KCl, and CaCl2. 

Molal concentrations (mol salt/kg water). Weight percent PEG. 

Stock solutions are typically prepared on a weight percent (PEGs or salts), molar (salts), or molal (salts) concentration basis. Equal aliquots by volume of each stock solution are combined and mixed to yield an ABS. The total system composition can be calculated and the phase compositions taken from the appropriate phase diagram and tie line data. This method of reporting allows the results to be more readily compared with solvent extraction data. 

Figure 6 Pertechnetate distribution ratios in several ABSs versus the molal concentration of salt stock solution used to prepare the biphase with 40% PEG-2000.

This equation predicts a linear relationship between the logarithm of the retention factor and the molal salt concentration, which is indeed commonly observed. Figure 13.1 shows the retortion behavior of several proteins on a silica-based polar bonded phase, propylacetamide. The eluents were various concentrations of ammonium sulfate in 0.1 M phosphate buffer, pH 7. The slope of the relationship is 2-2.S (LAnoI) for this stationary phase. For n-butyl-and phenyl-derivatized silica-based phases, values of between 1 and 2 (LAnoO were observed (5). 

Figare 13.1 Dependence of the retention factor on the molal salt concentration. The stationary phase is propylacetamide bonded to a silica with an average pore size of 25 nm. Mobile phase ammonium sulfate in 0.1 M phosphate buffer, pH 7. (Reprinted from Ref. 10, p. 3232, by courtesy of Marcel Dekker, New York, 1990.)... 

Melander, Horv th and co-workers (7-9) have treated this phenomenon quantitatively. They derived an equation of the following form for the dependence of the retention factor on the molal salt concentration m, in HIC ... 

Tabulated values of solubilities of ionic salts refer to the maximum amount of solid that will dissolve in a given mass of water to give a saturated solution. Solubilities may also be expressed in concentrations, molalities or mole fractions. 

Finally, as an example of a highly soluble salt, we may take sodium chloride at 25° the concentration of the saturated solution is 6.16 molal. The activity coefficient of NaCl, like that of NaBr plotted in Fig. 72, passes through a minimum at a concentration between 1.0 and 1.5 molal and it has been estimated2 that in the saturated solution the activity coefficient has risen to a value very near unity. Writing y = 1.0, we find that the work required to take a pair of ions from the surface of NaCl into pure water at 25° has the rather small value... 

The search for a suitable electrolyte requires comprehensive studies. It is necessary to measure the conductivities of electrolytes with various solvents, solvent mixtures, and anions over the accessible concentration range of the salts, and to cover a sufficiently large temperature range and the whole composition range of the binary (or ternary) solvent mixture. Figure 11 shows, as an example, the conductivity plot of LiAsF6/GBL as a function of temperature and molality. 

Can the species activity coefficients be calculated accurately An activity coefficient relates each dissolved species concentration to its activity. Most commonly, a modeler uses an extended form of the Debye-Hiickel equation to estimate values for the coefficients. Helgeson (1969) correlated the activity coefficients to this equation for dominantly NaCl solutions having concentrations up to 3 molal. The resulting equations are probably reliable for electrolyte solutions of general composition (i.e., those dominated by salts other than NaCl) where ionic strength is less than about 1 molal (Wolery, 1983 see Chapter 8). Calculated activity coefficients are less reliable in more concentrated solutions. As an alternative to the Debye-Hiickel method, the modeler can use virial equations (the Pitzer equations ) designed to predict activity coefficients for electrolyte brines. These equations have their own limitations, however, as discussed in Chapter 8. 

The virial methods differ conceptually from other techniques in that they take little or no explicit account of the distribution of species in solution. In their simplest form, the equations recognize only free ions, as though each salt has fully dissociated in solution. The molality m/ of the Na+ ion, then, is taken to be the analytical concentration of sodium. All of the calcium in solution is represented by Ca++, the chlorine by Cl-, the sulfate by SO4-, and so on. In many chemical systems, however, it is desirable to include some complex species in the virial formulation. Species that protonate and deprotonate with pH, such as those in the series COg -HCOJ-C02(aq) and A1+++-A10H++-A1(0H), typically need to be included, and incorporating strong ion pairs such as CaSO aq) may improve the model s accuracy at high temperatures. Weare (1987, pp. 148-153) discusses the criteria for selecting complex species to include in a virial formulation. 

The semi-empirical Pitzer equation for modeling equilibrium in aqueous electrolyte systems has been extended in a thermodynamically consistent manner to allow for molecular as well as ionic solutes. Under limiting conditions, the extended model reduces to the well-known Setschenow equation for the salting out effect of molecular solutes. To test the validity of the model, correlations of vapor-liquid equilibrium data were carried out for three systems the hydrochloric acid aqueous solution at 298.15°K and concentrations up to 18 molal the NH3-CO2 aqueous solution studied by van Krevelen, et al. 

Data on the effect of temperature on salting out of ammonia are even less satisfactory than those for carbon dioxide. Perman obtained some data on a potassium sulfate solution at temperatures of 40° to 59°C and on two ammonium chloride solutions at temperatures from 19° to 58°C.(54). His ammonia concentrations were in the range of 5 to 13 molal. His data indicate only small changes in the salting-out coefficient, but the coefficient for ammonium chloride increases with temperature, which is contrary to the effect found with carbon dioxide. 

Coefficient expressing the effect of concentration of gas on its activity coefficient, kg H20/mole Coefficient expressing the effect of change of partial molal volume of electrolyte (with temperature) on the salting-out coefficient, kg H20/cm3. Salt concentration, mol/2. 

Ggawa, T. and Satoh, K. Density, partial molal volume, refractiye index, polarizability, and viscosity of concentrated and saturated aqueous solutions of Rochelle salt, J. Chem. Eng. Data, 21(l) 33-35, 1976. 

The total concentration of ions is 4 times that of the salt. When calculating the change in freezing point or hoUing point, the concentration of all the solute particles must be used, whether they are molecules or ions. The concentration of the ions in this solution of AlBtj is 1.072 molal, and this molality would be used to calculate A 7 and A 7. ... 

The surface tension of an aqueous solution containing an organic solvent (or salt) is a function of the molal solvent (or salt) concentration, m, and can be given by... 

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