In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and significantly increases the concentration of hydronium ions, whereas the weak acid only partially dissociates. Similarly, in a mixture of two weak acids, the acid that is relatively stronger produces more hydronium ions than the weaker acid.
In both cases, the dissociation of the weaker acid is suppressed when it is in the presence of a stronger acid. Le Châtelier’s principle explains that the formation of hydronium ions by the stronger acid shifts the equilibrium towards the reactants, thereby reducing the dissociation of the weak acid.
Thus, the pH of a mixture of acids is mainly determined by the concentration of the stronger acid. For example, in a mixture that contains 0.15 molar hydrochloric acid and 0.30 molar hydrocyanic acid, hydrochloric acid–a strong acid–produces a hydronium ion concentration of 0.15 molar.
In contrast, hydrocyanic acid–a weak acid–only partially dissociates. The concentration of hydronium ions produced by hydrocyanic acid can be calculated from its acid dissociation constant, Ka, and an ICE table.
The initial concentration of hydronium ions is equal to the initial concentration of hydrochloric acid, 0.15 molar, and the initial concentration of cyanide ions is zero. The change in concentration of hydronium ions and cyanide ions is denoted by x.
As x is a relatively small number, 0.30 minus x can be approximated to 0.30, and 0.15 plus x can be approximated to 0.15 using the 5% rule.
The Ka for hydrocyanic acid is 4.9 × 10−10, and it is equal to the concentration of hydronium ions times the concentration of cyanide ions divided by the concentration of hydrocyanic acid.
Substituting the values from the ICE table into the Ka expression gives the concentration of hydronium ions produced by hydrocyanic acid, which is negligible compared to the concentration of hydronium ions produced by hydrochloric acid.
The pH can be calculated by taking the negative log of the concentration of hydronium ions: 0.15 molar. Therefore, the pH of the mixture is solely determined by the concentration of hydrochloric acid, the strong acid.
Similarly, the pH of a mixture of two weak acids present in equal amounts will be primarily determined by the concentration of the relatively stronger acid.
For example, in a mixture of hydrofluoric acid and hydrocyanic acid, hydrofluoric acid will be the major determinant of the mixture’s pH, as it has a Ka of 3.5 × 10−4, which is almost a million times higher than the Ka of hydrocyanic acid.
