pH Calculator
Start from
Molarity of the acid or base as made up
Acetic acid 1.8×10⁻⁵ (pKa 4.76), HF 6.8×10⁻⁴, HCN 6.2×10⁻¹⁰. A value from 1 to 20 is read as pKa.
Try an example
Result
pH 2.88
The solution is acidic (below 7 at 25 °C).
- pH
- 2.875
- pOH
- 11.125
- [H⁺] mol/L
- 0.001333
- [OH⁻] mol/L
- 7.504 × 10⁻¹²
- Percent ionized:
- 1.33 %
- √(Ka·C) shortcut:
- 0.001342 mol/L (within 5 %, fine here)
- pKa:
- 4.745
Working
Assumes 25 °C (Kw = 1.0 × 10⁻¹⁴, so pH + pOH = 14) and ideal behaviour, which is accurate for dilute solutions. Strong acids and bases are treated as fully dissociated.
Calculate the pH, pOH, [H⁺] and [OH⁻] of a strong or weak acid or base from its concentration (and Ka or Kb), or convert between pH, pOH and ion concentrations. Weak acids and bases are solved exactly, including the water's own ions, with the percent ionized shown.
Formula
Worked Example
Solving Ka = x² ÷ (0.1 − x) gives x = 0.001333 mol/L, so pH = −log(0.001333) = 2.88. The shortcut x ≈ √(Ka × C) = 0.001342 is within 5% here because only 1.3% of the acid ionizes.
Frequently Asked Questions
How do I calculate the pH of a strong acid?
A strong acid such as HCl dissociates completely, so [H⁺] equals its concentration and pH = −log₁₀(C): 0.01 M HCl has a pH of 2. For a strong base, find pOH = −log₁₀[OH⁻] and subtract it from 14.
Why isn't the pH of 10⁻⁸ M HCl equal to 8?
Because water itself supplies 10⁻⁷ M of H⁺, ten times more than the acid adds. Counting both gives [H⁺] ≈ 1.05 × 10⁻⁷ M and a pH of about 6.98, slightly acidic, as an acid solution must be.
What is the difference between Ka and pKa?
Ka is the acid dissociation constant and pKa = −log₁₀(Ka). A lower pKa means a stronger acid: acetic acid has a pKa of 4.76 and hydrofluoric acid 3.17. When pH equals pKa, the acid is exactly half ionized.