Equilibria in Solutions of Strong Electrolytes

Hydrochloric acid, HCl, is one of a half-dozen strong acids, which means that its acid ionization constant is too large to measure accurately. We must find a way to handle the activity of unionized species such as HCl in spite of their unmeasurably small concentrations. Since aqueous HCl has an appreciable vapor pressure we assume that aqueous unionized HCl in an aqueous solution of HCl is at equilibrium with gaseous HCl. From the fundamental fact of phase equilibrium 

The aqueous unionized HCl is also at equilibrium with H and Cl in the solution  

For the unionized HCl, we use a new molality description, in which m° is replaced by m, a small constant molality. We will not be able to determine the value of m, but will be able to eliminate it from our equations. We write 

Equation (7.4-9) does not mean that the molality of the unionized HCl is roughly equal to the product of the molalities of the ions, because m is not equal to m°. 

We can now relate the partial vapor pressure of gaseous HCl to the molality of the solution with which it is at equilibrium. If the vapor is assumed to be an ideal gas mixture. 

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T. F. Young, L. F. Maranville, and H. M. Smith, Raman Spectral Investigations of Ionic Equilibria in Solutions of Strong Electrolytes in W. J. Hamer, ed., The Structure of Electrolytic Solutions (New York Wiley, 1959). 

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