Acid-Base pH Calculator
Choose a calculation type, enter your values, and click Calculate. Assumes aqueous solution at 25°C.
For extremely dilute solutions, high ionic strength, or polyprotic systems, use full equilibrium methods in advanced chemistry software.
What this acid base calculator does
This calculator quickly estimates pH, pOH, [H⁺], and [OH⁻] for common introductory chemistry cases: strong acids, strong bases, weak acids, weak bases, and buffers. It’s designed for students, teachers, lab prep, and anyone who wants fast acid-base math without doing every logarithm by hand.
Quick refresher: pH and pOH
Core definitions
- pH = -log₁₀[H⁺]
- pOH = -log₁₀[OH⁻]
- At 25°C: pH + pOH = 14
- At 25°C: [H⁺][OH⁻] = 1.0 × 10⁻¹⁴
A pH below 7 is acidic, above 7 is basic, and around 7 is neutral (for pure water at 25°C).
How each mode is calculated
1) Strong acid
For a monoprotic strong acid, dissociation is assumed complete, so [H⁺] ≈ C, where C is the formal acid concentration.
2) Strong base
For bases that release one hydroxide per formula unit, [OH⁻] ≈ C. Then pOH is calculated first, and pH follows from 14 - pOH.
3) Weak acid
Uses the equilibrium expression with the exact quadratic solution: Ka = x²/(C - x), where x = [H⁺].
4) Weak base
Similar approach: Kb = x²/(C - x), where x = [OH⁻].
5) Buffer
Uses Henderson–Hasselbalch: pH = pKa + log₁₀([A⁻]/[HA]). This is especially useful for acetate, phosphate, and biological buffer systems.
Tips for better accuracy
- Use molar concentrations in mol/L.
- Check that Ka/Kb values are for the same temperature and solvent conditions.
- For very concentrated or very dilute solutions, ideal assumptions may break down.
- For polyprotic acids (e.g., H₂SO₄ in full detail), multi-step equilibria may be needed.
Worked examples you can try
Example A: Strong acid
Enter 0.01 M in Strong Acid mode. You should get pH ≈ 2.00.
Example B: Weak acid (acetic acid-like)
Enter C = 0.10 M and Ka = 1.8 × 10⁻⁵. You should get pH around 2.87.
Example C: Buffer
Set pKa = 4.76, [A⁻] = 0.20 M, [HA] = 0.10 M. pH should be about 5.06.
Final note
This acid base calculator is ideal for classwork, exam checking, and quick lab estimates. If your system includes multiple equilibria, salt effects, activity corrections, or non-aqueous solvents, use an advanced equilibrium solver for research-level precision.