Fraction and Concentration Diagrams

The equilibrium fractions of species are closely related to their ratio, so that it is not surprising that the fraction function has been used in the way we have used Rg above. The fractions are related to the equilibrium constant and H  

To construct a pH problem-solving method we proceed just as with the ratios and find the material balance expression for the fractions just derived from the equilibrium condition equation. From the derivation of equation (3-2) we have (C is the total of HX and X ) 

We have called these material balance functions the a fractions. They equal the equilibrium a fractions only at one value of H at which equilibrium and material balance conditions are simultaneously satisfied. This can be found graphically or programmed for computer solution. For pure acid and base, Cj, or C zero, a simple, invariant shaped curve results in the logarithmic plots. 

For pure acid solutions, with = 0, we get from equation (4-4) 

Example 1. Find the pH of 0.010 M acetic acid, and also of 0.010 M sodium acetate. 

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