Question

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

Answer 1

Answer:

The answer is in the explanation.

Explanation:

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.

Answer 2

The dissociation of a weak acid can be ignored when calculating the pH of a solution because; the dissociation of the acid is small enough that the change in the concentration of the acid as the reaction comes to equilibrium can be ignored.

What is dissociation of an Acid?

The reason why the dissociation of a weak acid can be ignored when calculating the pH of a solution is because the dissociation of the acid is small enough that the change in the concentration of the acid as the reaction comes to equilibrium can be ignored.

Now, the acid must be weak enough that ΔC is very small when compared with the initial concentration of the acid. However, the acid must be strong enough such that the H₃O⁺ ions from the acid overwhelm the dissociation of water.

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