Science
Ph calculations.
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 · 5 phases
Step by step.
- Identify whether the substance is a strong or weak acid/base.
- For strong acids, [H⁺] equals the acid concentration; calculate pH = −log[H⁺].
- For strong bases, calculate [OH⁻], then pOH = −log[OH⁻], then pH = 14 − pOH.
- For weak acids, set up the Ka equilibrium expression and solve for [H⁺].
- Convert [H⁺] to pH using pH = −log[H⁺].
Examples
Real-world.
- 1 A 0.01 M HCl solution has pH = −log(0.01) = 2
- 2 A 0.1 M NaOH solution has pOH = 1, so pH = 13
- 3 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.