pH Formula
The pH formula pH = -log[H⁺] measures the acidity or basicity of a solution.
Learn to calculate pH and hydrogen ion concentration with examples.
The Formula
pH = -log[H⁺]
The pH scale measures how acidic or basic a solution is. It ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral.
Variables
| Symbol | Meaning |
|---|---|
| pH | The pH value (dimensionless, scale from 0 to 14) |
| [H⁺] | Concentration of hydrogen ions (measured in moles per liter, mol/L) |
| log | Base-10 logarithm |
Example 1
A solution has a hydrogen ion concentration of 0.001 mol/L. What is its pH?
[H⁺] = 0.001 = 10⁻³ mol/L
pH = -log(10⁻³) = -(-3)
pH = 3 (acidic)
Example 2
A solution has a pH of 5.5. What is the hydrogen ion concentration?
Rearrange: [H⁺] = 10⁻ᵖᴴ
[H⁺] = 10⁻⁵·⁵
[H⁺] ≈ 3.16 × 10⁻⁶ mol/L
When to Use It
Use the pH formula to measure or calculate acidity.
- Determining whether a solution is acidic (pH < 7), neutral (pH = 7), or basic (pH > 7)
- Converting between pH and hydrogen ion concentration
- Water quality testing and environmental science
- Biochemistry and medical applications (blood pH is about 7.4)