Stoichiometric molality

The Pseudo-Phase Model Consider a process in which surfactant is added to water that is acting as a solvent. Initially the surfactant dissolves as monomer species, either as molecules for a non-ionic surfactant or as monomeric ions for an ionic surfactant. When the concentration of surfactant reaches the CMC, a micelle separates from solution. In the pseudo-phase model,20 the assumption is made that this micelle is a separate pure phase that is in equilibrium with the dissolved monomeric surfactant. To maintain equilibrium, continued addition of surfactant causes the micellar phase to grow, with the concentration of the monomer staying constant at the CMC value. This relationship is shown in Figure 18.14 in which we plot m, the stoichiometric molality,y against mj, the molality of the monomer in the solution. Below the CMC, m = m2, while above the CMC, m2 = CMC and the fraction a of the surfactant present as monomer... 

Figure 18.14 Comparison of the stoichiometric molality m and the molality of the monomer m2 in a surfactant solution, according to the pseudo-phase model.

Since E is known, and the e.m.f. of the cell (E) can be measured with various concentrations of acid, sodium salt and sodium chloride, i.e., for various values of mi, rrh and m3 in the cell depicted above, it is possible to evaluate the left-hand side of equation (14) or (15). In dilute solution, the sodium chloride may be assumed to be completely dissociated so that the molality of the chloride ion can be taken as equal to that of the sodium chloride, i.e., mcr is equal to m3. The acid HA will be partly in the undissociated form and partly dissociated into hydrogen and A ions the stoichiometric molality of HA is mi, and if nin is the molality of the hydrogen ions resulting from dissociation, the molality of undissociated HA molecules, i.e., maA in equation (15), is equal to mi — mn. Finally, it is required to knowm rriA- the A ions are produced by the dissociation of NaA, which may be assumed to be complete, and also by the small dissociation of the acid HA it follows, therefore, that mA is equal to m2 -f Since mu, the hydrogen ion concen-... 

If the stoichiometric molality of II2A is m in a given solution and that of the salt NaHA, assumed to be completely dissociated into HA"" ions, is m2, then... 

In these equations, v = Vff + vx, where v is the number of ions z is the charge on the ion, I is the (stoichiometric) molal ionic strength, and other terms are explained below. 

If we take the quantity of solvent as being equal to 1/3/q, m is equal to the stoichiometric molality of the electrolyte. Thus... 

These are examples of geometric means. For a 1-3 electrolyte such as CrCls, m is equal to 21) / m2 = 2.2795m2, where m2 is the stoichiometric molality of the solute (the molality that would occur if no dissociation occurred). The chemical potential of the neutral electrolyte solute is given by... 

At 25 C a solution of acetic acid with a stoichiometric molality of 0.100molkg is approximately 1.32% ionized. Assuming that this percentage ionization apphes at the freezing temperature, find the freezing temperature of this solution. 

Write expressions for y and m for Mg3(P04)2 in terms of the stoichiometric molality and the activity coefficients of the ions. Neglect hydrolysis (a poor approximation). 

Here y+ is the mean molal activity coefficient, and m c is the total (stoichiometric) molal concentration of HCl in solution. " Combining Eqs. (6)-(9), and assuming yhci = 1, we obtain,... 

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