Calculating the pH of Weak Acid Solutions

Since a weak acid dissolved in water can be viewed as a prototype of almost any equilibrium occurring in aqueous solution, we will proceed carefully and systematically. Although some of the procedures we develop here may seem superfluous, they will become essential as the problems become more complicated. We will develop the necessary strategies by calculating the pH of a 1.00 M solution of HF (Ka = 7.2 X 10-4). 

The first step, as always, is to write the major species in the solution. From its small Ka value, we know that hydrofluoric acid is a weak acid and will be dissociated only to a slight extent. Thus when we write the n jor 

Just as with gas-phase equilibria, the equilibrium constant expressions for reactions in solution actually require activities. In all our calculations, we will assume that ax = [x], where x represents a species in solution. 

The next step is to decide which of the major species can furnish H+ ions. Actually, both major species can do so  

In aqueous solutions typically one source of H+ can be singled out as dominant. By comparing FCa for HF with Kw for H20, we see that hydrofluoric acid, although weak, is still a much stronger acid than water. Thus we will assume that hydrofluoric acid will be the dominant source of H+. We will ignore the tiny contribution expected from water. 

Therefore, it is the dissociation of HF that will determine the equilibrium concentration of H and hence the pH  

---

Calculating the pH of Weak Acid Solutions The pH of a Mixture of Weak Acids Percent Dissociation ... 

---