Advanced Placement (AP) Chemistry Practice Exam 2025 - Free AP Chemistry Practice Questions and Study Guide

The pH of a solution is fundamentally determined by the concentration of hydronium ions, which is represented as [H₃O⁺] or [H⁺] in many contexts. The pH scale is logarithmic and defined by the negative logarithm of the hydronium ion concentration:

\[ \text{pH} = -\log[H₃O⁺] \]

This relationship means that as the concentration of hydronium ions increases, the pH of the solution decreases, indicating a more acidic solution. Conversely, a decrease in hydronium concentration results in a higher pH, characteristic of basic solutions.

The other options, while related to pH, do not directly define it. The concentration of hydroxide ions is relevant in discussions of pH, particularly in calculating the pOH and connecting to the relationship between acids and bases in aqueous solution. The balance between hydronium and hydroxide ions is critical, but ultimately, pH is derived from the hydronium ion concentration.

Considering both acids and bases is important for understanding how they interact in solution, but the pH itself is specifically a measure that reflects the hydronium ion presence. Similarly, while temperature can influence