Explain why the dissociation of a weak acid can be ignored when calculating the pH of a solution that contains both a weak acid
and a strong acid. The ion from the strong acid is a common ion in the dissociation reaction of the weak acid. Consequently, it shifts the equilibrium back toward the which the dissociation of the to the point where the amount of ion that it produces is negligible
The dissociation of a weak acid consist in the following equilibrium:
HX ⇄ H⁺ + X⁻
Where Ka is defined as:
Ka = [H⁺] [X⁻] / [HX]
A strong acid (HY) dissociates completely in water, thus:
HY → H⁺ + Y⁻
As the strong acid produces H⁺, in the equilibrium, the reaction shifts to the left -The undissociated form-, reducing the production of H⁺, allowing ignore the dissociation of the weak acid when calculating the pH.
