Library / Foundations: Chemistry / FCH-23
Foundations: Chemistry
pH of a solution
How acidic or alkaline a solution is, from the hydrogen ion concentration.
- Ref
- FCH-23
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- pH
[H⁺] in mol/dm³. For a strong acid [H⁺] equals the acid concentration times the number of H⁺ it gives. At 25 °C, pH + pOH = 14 and Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴.
Each pH step is a tenfold change in the hydrogen ion concentration, so small changes in pH are big changes in acidity.
Calculator
mol/dm³
- pOH11
- [OH⁻]1×10−11 mol/dm³
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
pH () current value 3
[H⁺]Hydrogen ion concentrationKey driver
014
- Each unit of pH is a factor of ten: pH 3 is ten times more acidic (in [H⁺]) than pH 4.
- Neutral water at 25 °C has pH 7, where [H⁺] = [OH⁻] = 10⁻⁷ mol/dm³.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| [H⁺] Hydrogen ion concentration | 1e-14 to 1 mol/dm³ | 0 | 14 | 14 | +10% | −0.0414 |
Sweep of Hydrogen ion concentration
| Hydrogen ion concentration (mol/dm³) | pH |
|---|---|
| 1e-14 | 14 |
| 5.623e-13 | 12.25 |
| 3.162e-11 | 10.5 |
| 1.778e-9 | 8.75 |
| 1e-7 | 7 |
| 5.623e-6 | 5.25 |
| 3.162e-4 | 3.5 |
| 0.01778 | 1.75 |
| 1 | 0 |
Sweep one variable
pH ()Hover, touch, or use the arrow keys on the chart
Variables and how each one contributes
pH
pH
What the equation gives you. Everything else feeds it.
[H⁺]
Hydrogen ion concentration
Concentration of hydrogen ions. Raising [H⁺] tenfold lowers pH by 1.
Worked example with the values above
- log₁₀(0.001) = −3.
- Change the sign.
- pH = 3.
Assumptions and limits
- Dilute solution.
- 25 °C for the pH + pOH = 14 relationship.
Common mistakes
- Forgetting the minus sign.
- Treating pH as linear in concentration.
Statistics and uncertainty
Because pH is logarithmic, a ±0.02 pH reading corresponds to about ±5% in [H⁺].
Where it is used
- Water and process chemistry control.
- Corrosion and battery electrolyte checks.
Maths you need
- Logarithms: log₁₀
- Powers of ten: standard form