3.7: Mixtures of Acids

The pH of a solution containing an acid can be determined using its acid dissociation constant and initial concentration. If a solution contains two different acids, then its pH can be determined using one of several methods depending on the relative strength of the acids and their dissociation constants.

In a strong and weak acid mixture, the strong acid dissociates completely and becomes a source of almost all the hydronium ions present in the solution. In contrast, the weak acid shows partial dissociation and produces a negligible concentration of hydronium ions. The high concentration of hydronium ions produced by the strong acid further reduces the dissociation of the weak acid. According to Le Chatelier's principle, this happens: "When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance." The excess hydronium ions produced by the strong acid disturb the equilibrium, and thus, the reaction will move in the reverse direction until the equilibrium is established. This leads to a decrease in the dissociation of the weak acid. Because of this decrease, the pH of a strong and weak acid mixture can be calculated from the concentration of the strong acid only. For example, the pH of a mixture with an equal concentration of hydrochloric acid (HCl), a strong acid, and formic acid (HCHO₂), a weak acid, can be determined from the concentration of HCl only. If the concentration of the HCl in the mixture is 0.0020 M, its pH can be calculated as follows.

pH = −log(0.002) = 2.7

Here, the concentration of hydronium ions produced by HCHO₂ and the autoionization of water is negligible and thus can be ignored.

A Mixture of Two Weak Acids with Different Dissociation Constants

In a mixture of two weak acids, the pH of a mixture will be determined by the stronger acid if its dissociation constant is significantly higher than the weaker acid. For example, in a mixture with an equal concentration of nitrous acid (HNO₂) and hypochlorous acid (HClO), the HNO₂ will be the main determinant of the pH of the mixture as its K_a (4.6 × 10⁻⁴) is approximately 10,000 times higher than the K_a (2.9 × 10⁻⁸) of HClO. According to Le Chatelier's principle, HClO shows decreased dissociation in the presence of HNO₂.

In a mixture of strong and weak acid, the strong acid dissociates completely, significantly elevating H₃O⁺ ion concentration, while the weak acid only partially dissociates.

Le Châtelier's principle explains that the stronger acid suppresses the weaker acid's dissociation by shifting the equilibrium towards the reactants.

Consider a mixture of 0.15 M HCl and 0.30 M HCN. The strong acid yields a H₃O⁺ concentration of 0.15 M, making the H₃O⁺ concentration from the weak acid negligible.

So, the pH in a mixture of acids is predominantly dictated by the concentration of the strong acid.

Similarly, when a mixture of two weak acids of equal amounts is present, the relatively stronger acid is the primary pH determinant.

For example, in a mixture of HF and HCN, HF will be the major determinant of the mixture's pH, as it has a K_a of 3.5 × 10⁻⁴ which is almost a million times higher than the K_a of hydrocyanic acid.