About this calculator
Weak acids such as acetic acid only partly ionise; give the concentration and Ka to solve the equilibrium.
How to use it
- Choose what you know.
- Enter the value.
- Read pH, pOH and both concentrations.
The formula
pH = −log₁₀[H⁺]; pH + pOH = 14; weak acid: [H⁺] = (−Ka + √(Ka² + 4Ka·C)) ÷ 2
- [H⁺]
- hydrogen ion concentration, mol/L
- Ka
- acid dissociation constant
Worked example
0.1 M acetic acid, Ka = 1.8 × 10⁻⁵
- [H⁺] = 1.33 × 10⁻³ mol/L.
- pH = 2.88; only 1.3% of the acid ionises.
What the result means
Lemon juice is about pH 2, pure water 7, baking soda solution about 8.3 and bleach about 12.
Assumptions
- 25 °C, where Kw = 1.0 × 10⁻¹⁴.
Limitations
- Very concentrated solutions need activities rather than concentrations.
Frequently asked questions
What is the pH of 0.01 M HCl?
2, since a strong acid ionises fully.
What is pOH?
−log₁₀[OH⁻]; pH + pOH = 14 at 25 °C.
Can pH be negative?
Yes, for very concentrated strong acids.
Last reviewed on 6 October 2026. Found a mistake? Tell us.