Acid Base Strength Table

Interactive reference and calculator for acid-base equilibrium constants (pKa, pKb, Ka, Kb). Compare relative strengths, estimate pH for weak acids/bases, and convert between logarithmic and exponential forms. Works 100% offline with unit toggles (M, mM, µM).

Educational only. Follow your lab SOPs and SDS.

Strength Calculator

Species Type

Weak Acid Weak Base Conjugate Pair

Equilibrium Constant

Enter Ka, Kb, pKa, or pKb. Accepts decimals (4.75) or scientific notation (1.8e-5).

Concentration (optional for pH)

Auto-calculate pH/pOH (for weak species)

Results

Acid Strength (pKa)

4.75

Base Strength (pKb)

9.25

1.78 × 10^-5

5.62 × 10^-10

Estimated pH/pOH (weak, 0.1 M)

2.88 pH

11.12 pOH

1.32e-3 M [H⁺]

Strength Classification

Weak acid (typical organic acid)

Acid	pKa	Base	pKb	Use

Understanding Acid-Base Strength

This tool helps you compare and convert between different measures of acid-base strength, essential for predicting reaction directions, designing buffers, and estimating pH in lab solutions.

How to Use This Tool

Select the species type: Weak Acid, Weak Base, or Conjugate Pair (to see both sides).
Enter a known constant. You can input pKa, pKb, Ka, or Kb. Use decimal (4.75) or scientific notation (1.8e-5).
Optionally add a concentration to estimate pH/pOH for weak species.
Results update instantly, showing all four constants, pH estimate, and a strength classification.
Use the reference table below to compare with common acids/bases.

Examples to Try

Acetic acid: Enter pKa = 4.75, concentration 0.1 M. Observe pH ≈ 2.88, Ka = 1.78×10⁻⁵.
Ammonia: Switch to Weak Base, enter pKb = 4.75, concentration 0.1 M. Observe pOH ≈ 2.88, pH ≈ 11.12.
Strong acid: Enter pKa = -2 (Ka = 100). Notice the warning about concentration assumptions.
Conjugate pair: Select Conjugate Pair, enter Ka = 6.2×10⁻¹⁰ (pKa 9.21). See the complementary pKb = 4.79.

Common Mistakes & Pitfalls

Ignoring concentration: pH of weak acids depends on concentration; at very low concentrations, water autoionization matters.
Misapplying approximations: The 5% rule: if [H⁺] > 5% of total concentration, the simple quadratic should be solved.
Unit confusion: Ensure concentration units match (M, mM, µM). The tool converts internally to molar (M).
Strong vs. weak: pKa < 0 or pKb < 0 indicates strong species; pH calculation assumes complete dissociation.

Quick Comparison: Effect of pKa on pH

Adjust pKa for a weak acid at fixed concentration (0.01 M) to see how strength influences pH.

pKa Value: 5.0 Weak acid (0.01 M)

Estimated pH

3.50

[H⁺]

3.16e-4 M

Dissociation %

3.16%

Strength

Moderate

Frequently Asked Questions

What are the accuracy limits of the pH estimate?

For weak acids/bases, the simple approximation [H⁺] = √(Ka·C) is used if dissociation < 5%. If the tool detects greater dissociation, it warns you to solve the quadratic. Activity coefficients and ionic strength are not considered.

When do the approximations fail?

For very dilute solutions (< 10⁻⁵ M), water autoionization contributes significantly. For very strong acids/bases (pKa < -2 or pKb < -2), complete dissociation is assumed but actual activity differs. Always verify with measurement.

How should I measure these values in the lab?

Use a calibrated pH meter with appropriate buffers. For pKa determination, perform a titration and use the Henderson-Hasselbalch equation at half‑equivalence. Temperature control is critical—constants are temperature‑dependent.

What safety notes apply?

Strong acids/bases cause severe burns. Always add acid to water, not water to acid. Use PPE (gloves, goggles, lab coat) and work in a fume hood when handling concentrated solutions. Neutralize waste before disposal per your institutional SOPs.

Formulas & Assumptions

pKa = -log₁₀(Ka), pKb = -log₁₀(Kb)
Ka × Kb = Kₐ = 10⁻¹⁴ at 25°C (for conjugate pairs in water)
Weak acid pH: [H⁺] = √(Ka·C) if [H⁺] < 0.05·C, else quadratic solution
Weak base pOH: [OH⁻] = √(Kb·C) with same 5% rule
Constants: Kₐ = 1.0×10⁻¹⁴ (at 25°C), R = 8.314 J·mol⁻¹·K⁻¹ (not used here)
Rounding: Results rounded to 3 significant figures; pH/pOH to 2 decimal places.

Menu

Acid Base Strength Table

AI Assistant

Was this tool helpful?