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

The formula used to calculate pH is based on the concentration of hydrogen ions (H⁺) in a solution. pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration. This relationship is expressed mathematically as pH = -log[H⁺].

This means that as the concentration of hydrogen ions increases, the pH value decreases, indicating a more acidic solution. Conversely, a lower concentration of hydrogen ions corresponds to a higher pH, indicating a more basic solution. It is a crucial concept in understanding the acidity or basicity of solutions in chemistry and is fundamental in acid-base chemistry, particularly for titration and equilibrium reactions.

The other formulas provided involve hydroxide ion concentration, acid dissociation constants, and base dissociation constants, which relate to different aspects of acid-base balance but do not provide the direct measure of pH as defined by hydrogen ion concentration.