15.8
Mixtures of Acids
The pH of a solution containing an acid can be determined using its acid dissociation constant and its initial concentration. If a solution contains t…
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.
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Q1: Why does a strong acid suppress the dissociation of a weak acid in a mixture?
According to Le Châtelier's principle, when a strong acid produces excess hydronium ions, it shifts the weak acid's equilibrium toward reactants, reducing its dissociation. The high hydronium concentration disturbs the equilibrium, causing the reaction to reverse until balance is restored. This suppression effect means the weak acid contributes negligibly to the overall pH.
Q2: How do you calculate the pH of a mixture containing a strong acid and a weak acid?
Calculate pH using only the strong acid's concentration, since it produces nearly all hydronium ions. For example, a mixture with 0.15 M hydrochloric acid and 0.30 M hydrocyanic acid has pH determined solely by the 0.15 M strong acid concentration. The weak acid's contribution is negligible due to suppressed dissociation.
Q3: What role does the acid dissociation constant play when comparing two weak acids?
The acid dissociation constant (Ka) determines which weak acid dominates pH in a mixture. The acid with the significantly higher Ka value produces more hydronium ions and controls the mixture's pH. For instance, hydrofluoric acid with Ka = 3.5 × 10−4 is nearly a million times stronger than hydrocyanic acid with Ka = 4.9 × 10−10.
Q4: How can you use an ICE table to find hydronium ion concentration from a weak acid in a mixture?
Set up an ICE table with initial concentrations, changes denoted by x, and equilibrium values. Apply the 5% rule to simplify calculations when x is small. Substitute equilibrium concentrations into the Ka expression to solve for x, which represents the hydronium ions produced by the weak acid.
Q5: In a mixture of two weak acids with equal concentrations, which acid determines the pH?
The relatively stronger weak acid, with the higher Ka value, determines the pH. In a mixture of nitrous acid (Ka = 4.6 × 10−4) and hypochlorous acid (Ka = 2.9 × 10−8), nitrous acid controls pH because its Ka is approximately 10,000 times larger, suppressing the dissociation of the weaker acid.
Q6: Why is the contribution of a weak acid to total hydronium concentration often negligible in acid mixtures?
The weak acid's partial dissociation is further suppressed by hydronium ions from the stronger acid. This suppression, explained by Le Châtelier's principle, reduces the weak acid's dissociation so significantly that its hydronium contribution becomes negligible compared to the stronger acid's contribution.
Q7: What is the relationship between acid strength and pH determination in mixtures?
The strongest acid in a mixture dominates pH calculation because it produces the highest hydronium ion concentration. Whether mixing a strong with a weak acid or two weak acids, the acid with the highest dissociation constant or complete dissociation controls the overall pH of the solution.