pH Calculator
Calculate pH, pOH, [H⁺], [OH⁻], and percent dissociation for strong and weak acids or bases. This upgraded calculator is built to be more student-friendly, more visual, and more educational, with guided modes, quick picks, interpretation, and step-by-step chemistry.
Background
Strong acids and bases dissociate almost completely in water, while weak acids and weak bases establish an equilibrium. For weak species, this calculator uses the exact quadratic solution rather than relying only on the √(K·C) shortcut. That makes the result more reliable and also helps students understand when the weak-acid or weak-base approximation is acceptable.
How to use this calculator
- Strong Acid / Base: Enter concentration and (optionally) stoichiometric factor n.
- Weak Acid / Base: Enter concentration and Ka/pKa or Kb/pKb. The calculator solves the equilibrium exactly.
- pH ↔ pOH / Ions: Convert between pH, pOH, [H⁺], and [OH⁻].
- Percent Dissociation: Understand how much of a weak species actually ionizes.
Assumes 25 °C, so pH + pOH = 14. Activity effects are ignored.
How this calculator works
- Strong acids/bases: [H⁺] = n·C or [OH⁻] = n·C.
- Weak species: solves K = x² / (C − x) using the quadratic formula.
- pH is computed from pH = −log₁₀([H⁺]).
- pOH is computed from pOH = −log₁₀([OH⁻]).
- Percent dissociation: (x / C) × 100%.
Formula & Equations Used
pH definition: pH = −log₁₀([H⁺])
pOH definition: pOH = −log₁₀([OH⁻])
Relationship: pH + pOH = 14
Weak acid equilibrium: Ka = x² / (C − x)
Quadratic solution: x = (−K + √(K² + 4KC)) / 2
Percent dissociation: (x / C) × 100%
Example Problems & Step-by-Step Solutions
Example 1 — Strong acid
0.100 M HCl:
- Strong acid → complete dissociation
- [H⁺] = 0.100
- pH = −log(0.100) = 1.00
Example 2 — Weak acid
Acetic acid, 0.10 M, pKa = 4.76:
- Convert pKa → Ka ≈ 1.74×10⁻⁵
- Solve quadratic for x
- Find pH from [H⁺] = x
- Result: pH ≈ 2.87
Example 3 — Weak base
NH₃, 0.10 M, pKb = 4.75:
- Convert pKb → Kb
- Solve for [OH⁻]
- Find pOH and then pH
- Result: pH ≈ 9.25
Frequently Asked Questions
Q: When is √(K·C) valid?
When the dissociation fraction is small (about 5% or less). This calculator always solves the quadratic and shows you whether the shortcut is reasonable.
Q: What does percent dissociation mean?
It tells you what fraction of molecules actually ionize. Weak acids and bases typically have low percent dissociation.
Q: Can I use this for polyprotic acids?
This calculator models one dissociation step at a time. That works well for many strong acids/bases and common weak-acid or weak-base classroom problems, but full multi-step systems such as polyprotic acids need a more advanced equilibrium tool.
Q: Does temperature matter?
Yes. This calculator assumes 25 °C. At other temperatures, pH + pOH ≠ 14.
