Ph calculations

Science

Definition

pH calculations involve determining the acidity or basicity of a solution using the formula pH = −log[H⁺]. For strong acids and bases, [H⁺] or [OH⁻] can be calculated directly from concentration. For weak acids/bases, equilibrium expressions (Ka or Kb) are needed.

How It Works

  1. Identify whether the substance is a strong or weak acid/base.
  2. For strong acids, [H⁺] equals the acid concentration; calculate pH = −log[H⁺].
  3. For strong bases, calculate [OH⁻], then pOH = −log[OH⁻], then pH = 14 − pOH.
  4. For weak acids, set up the Ka equilibrium expression and solve for [H⁺].
  5. Convert [H⁺] to pH using pH = −log[H⁺].

Examples

  • A 0.01 M HCl solution has pH = −log(0.01) = 2
  • A 0.1 M NaOH solution has pOH = 1, so pH = 13
  • Calculating the pH of a 0.1 M acetic acid solution using Ka = 1.8 × 10⁻⁵
Key Fact

pH = −log[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14 at 25°C.

Related Terms

Ph scale Acid-base theory Buffers Strong vs weak acids

Study This Concept

Practice pH calculations with free review games in these units:

AP Chemistry Unit 8: Acids and Bases