Advanced Placement (AP) Chemistry Practice Exam

What factor primarily determines the dissociation of weak acids in solution?

1. Electrolytic nature.

2. Concentration of solute.

3. Presence of other acids.

4. Acidity constant (Ka).

The correct answer is: Acidity constant (Ka).

The dissociation of weak acids in solution is primarily determined by the acidity constant, known as Ka. The acidity constant is a quantitative measure of the strength of an acid in solution, reflecting how readily it donates protons (H⁺ ions) to the solution. When a weak acid dissociates, it establishes an equilibrium between the undissociated acid and its ions. The value of Ka indicates the extent to which the acid will dissociate: a larger Ka means that the acid dissociates more completely, while a smaller Ka shows that the acid remains largely undissociated. The concentration of the weak acid itself can influence the position of the equilibrium, but it is the inherent strength of the acid, as shown by its Ka value, that fundamentally controls the degree of dissociation. This means that even as the concentration changes, the ability of the acid to dissociate depends on its properties as defined by its acidity constant. Other factors, such as the presence of other acids, could potentially impact the overall behavior of acids in solution, but again, the intrinsic dissociation of a weak acid is primarily determined by its Ka. Thus, understanding the concept of the acidity constant is crucial for predicting the behavior of weak acids in various conditions