Mixtures of Acids

The strong acid dissociates completely, producing a high concentration of hydronium ions that shifts the weak acid's equilibrium toward reactants, suppressing its dissociation. For example, in a 0.15 M HCl and 0.30 M HCN mixture, HCl yields 0.15 M H3O+, making the weak acid's contribution negligible. Therefore, pH is determined almost entirely by the strong acid's concentration.

When a strong acid produces excess hydronium ions, it disturbs the weak acid's equilibrium. According to Le Châtelier's principle, the system shifts in the reverse direction to minimize this disturbance, reducing the weak acid's dissociation. This excess hydronium concentration effectively suppresses the weaker acid's ionization, making its pH contribution negligible.

The relatively stronger weak acid controls the pH when its dissociation constant is significantly higher than the other. For instance, in an HF and HCN mixture, HF dominates because its Ka of 3.5 × 10−4 is nearly a million times larger than HCN's Ka. The stronger acid's higher dissociation produces more hydronium ions, determining the overall pH.

Yes, pH can be calculated using only the strong acid's concentration because the weak acid's hydronium contribution is negligible. For a 0.0020 M HCl and formic acid mixture, pH = −log(0.002) = 2.7. The hydronium ions from the weak acid and water autoionization are suppressed and can be ignored in the calculation.

In a weak acid mixture, the acid with the significantly larger Ka value becomes the primary pH determinant. For example, HNO2 with Ka = 4.6 × 10−4 dominates over HClO with Ka = 2.9 × 10−8 because its dissociation constant is approximately 10,000 times higher. The stronger acid's greater ionization produces more hydronium ions, controlling the solution's pH.

Weak acid dissociation decreases significantly in the presence of a stronger acid due to the common ion effect and equilibrium shift. The stronger acid floods the solution with hydronium ions, pushing the weak acid's equilibrium leftward according to Le Châtelier's principle. This suppression makes the weak acid's contribution to pH negligible in the mixture.

The dissociation constant ratio determines which weak acid controls pH in a mixture. When one acid's Ka is significantly larger—such as HNO2's Ka being 10,000 times greater than HClO's—that acid ionizes much more extensively. The acid with the substantially higher Ka produces more hydronium ions and becomes the dominant pH determinant in the solution.