Test: Proton Transfer, Strong and Weak Acid - Year 11 MCQ

When propanoic acid is added to water, an equilibrium is established between the intact acid molecules and their ions. This equilibrium predominantly favors the left side, indicating a high concentration of intact acid molecules with a low concentration of H ions in the solution. Understanding this equilibrium is fundamental in comprehending the behavior of weak acids in aqueous solutions.

The concentration of an acid solution refers to the number of acid molecules present per unit volume of the solution. This means that a concentrated acid solution has a higher density of acid molecules compared to a dilute solution. It's important to note that concentration does not necessarily indicate the strength of the acid, as it could be from a weak acid that does not fully dissociate. Therefore, the acidity level of an acid solution is not solely determined by its concentration but also by factors like the degree of dissociation of the acid molecules.

The scenario where a dilute hydrochloric acid (HCl) solution can be more acidic than a concentrated ethanoic acid (CH3COOH) solution arises from the fact that most HCl molecules dissociate in solution compared to CH3COOH. This higher degree of dissociation leads to a greater release of hydrogen ions, making the HCl solution more acidic despite its dilute nature. In contrast, even though the ethanoic acid solution may be concentrated, its weaker tendency to dissociate limits the release of acidic hydrogen ions, resulting in lower acidity compared to the HCl solution.

A dilute solution of a strong acid can be more acidic than a concentrated solution of a weak acid because strong acids have a higher tendency to dissociate readily, releasing more hydrogen ions into the solution even at lower concentrations. This increased dissociation of the strong acid contributes to higher acidity levels, making the dilute solution more acidic compared to the concentrated solution of a weak acid that may not fully dissociate. Therefore, the inherent strength of the acid in terms of its dissociation behavior plays a crucial role in determining the acidity of the solution, regardless of its concentration level.