📘 IB Understanding
The pH scale is a logarithmic measure of the hydrogen ion concentration [H+] in a solution. A change of one pH unit corresponds to a tenfold (10x) change in [H+].
What is pH?
pH stands for "power of hydrogen" and provides a convenient way to express the acidity or basicity of a solution on a scale typically ranging from 0 to 14 (at 25°C).
Key Formulas
- pH = -log10[H+]
- [H+] = 10-pH
- pOH = -log10[OH-]
- pH + pOH = 14.00 (at 298 K)
- Kw = [H+][OH-] = 1.0 x 10-14 (at 298 K)
The pH Scale
The Logarithmic Relationship
Because pH uses a log10 scale, each unit change represents a 10-fold change in [H+]:
| pH | [H+] / mol dm-3 | Classification |
|---|---|---|
| 1 | 0.1 (10-1) | Strongly acidic |
| 3 | 0.001 (10-3) | Acidic |
| 7 | 10-7 | Neutral (at 25°C) |
| 11 | 10-11 | Basic |
| 14 | 10-14 | Strongly basic |
Worked Example
What is the pH of a solution with [H3O+] = 2.5 x 10-3 mol dm-3?
pH = -log(2.5 x 10-3)
pH = 2.60
The Ionic Product of Water (Kw)
Pure water undergoes autoionisation:
H2O(l) ⇌ H+(aq) + OH-(aq)
At 25°C, Kw = [H+][OH-] = 1.0 x 10-14. Since [H+] = [OH-] in pure water, both equal 10-7, giving a neutral pH of 7.
⚠️ Exam Tip
Significant figures in pH: the number of decimal places in the pH value should match the number of significant figures in the [H+] concentration. For example, [H+] = 2.5 x 10-3 (2 s.f.) gives pH = 2.60 (2 decimal places).