Advanced Placement (AP) Chemistry Practice Exam

Which calculation would provide the equilibrium constant for the dissociation of a weak acid?

1. The pH of the solution.

2. The percent dissociation of the acid.

3. The concentration of OH- ions in solution.

4. The pKa value of the acid.

The correct answer is: The pKa value of the acid.

To determine the equilibrium constant for the dissociation of a weak acid, the pKa value is an essential parameter. The pKa is related to the equilibrium constant (Ka) through the relationship: \[ \text{pKa} = -\log(\text{Ka}) \] This means that if you know the pKa, you can easily calculate the Ka (the equilibrium constant) using the inverse logarithmic relationship. Since the pKa specifically quantifies the strength of an acid by indicating the degree to which it dissociates in solution, it directly provides a measure of the equilibrium condition of the acid dissociation reaction. While other factors such as pH, percent dissociation, and concentration of hydroxide ions can provide useful information about the acid and its behavior in solution, they do not directly yield the equilibrium constant without further calculations or conversions. For example, knowing the pH can allow for the calculation of hydrogen ion concentration, which is a step in finding the Ka, but it is not the most straightforward approach compared to utilizing the pKa. Thus, the pKa value is the most direct and useful measurement for determining the equilibrium constant for the dissociation of a weak acid.