🧪 pKa Calculator
Quickly convert Ka ↔ pKa and estimate weak acid properties.
Enter either Ka or pKa. If both are provided, pKa takes priority.
Provides pH and percent ionization using the square-root approximation (valid for weak acids).
How to Use This Calculator
Input Ka or pKa
Use experimental data or reference tables to provide either Ka or pKa. The calculator converts between the two.
Optional: add concentration
Enter the molar concentration of the weak acid to estimate [H⁺], pH, and percent ionization.
Calculate
Click “Calculate” to view Ka, pKa, and (if applicable) approximate equilibrium values for the weak acid solution.
Interpret the output
Use the results to compare acid strengths, prepare buffer calculations, or validate laboratory measurements.
Formula
pKa = −log₁₀(Ka)
Ka = 10−pKa
The square-root approximation for a weak monoprotic acid HA ⇌ H⁺ + A⁻ is:
[H⁺] ≈ √(Ka × Ca)
From which pH = −log₁₀[H⁺] and % ionization = ([H⁺]/Ca) × 100.
Example: Acetic acid
Ka = 1.8 × 10−5, pKa = 4.74
At 0.10 M: [H⁺] ≈ √(1.8 × 10−5 × 0.10) = 1.34 × 10−3 M
pH ≈ 2.87, % ionization ≈ 1.34%
Full Description
Acid dissociation constants define acid strength and are essential in analytical chemistry, biochemistry, and pharmaceutical formulation. This calculator streamlines Ka ↔ pKa conversions and expands on the data by estimating solution properties for weak acids. By adding optional concentration input, you can quickly model pH and ionization without solving quadratic equations each time.
The interface is designed for quick checks during titration planning, buffer design, or lecture demonstrations. While the approximations assume single-proton dissociation and low ionization, they provide accurate starting points for most classroom and laboratory scenarios.
Use it when
- Comparing acid strengths: Convert Ka values to pKa for easier comparison.
- Preparing buffer solutions: Estimate pH shifts before detailed Henderson–Hasselbalch calculations.
- Teaching equilibrium concepts: Demonstrate how Ka impacts pH and percent ionization.
- Validating experimental data: Cross-check Ka values derived from titration or spectrophotometry.
- Pharmaceutical development: Understand drug ionization at physiological pH.
Key benefits
- ✅ Instant conversions: Toggle between Ka and pKa instantly.
- ✅ Weak acid estimates: Get pH and percent ionization from a single concentration value.
- ✅ Accessible design: Clear layout for in-class demonstration or quick lab reference.
- ✅ Assumption transparency: Notes remind users when approximations are valid.
- ✅ Mobile compatible: Use on phones and tablets at the bench.
Frequently Asked Questions
Can I enter both Ka and pKa?
Yes, but pKa takes priority. The calculator uses pKa to compute Ka to avoid rounding inconsistencies.
What if the square-root approximation doesn't apply?
If percent ionization exceeds about 5%, the assumption x ≪ Ca fails. In such cases, solve the full equilibrium expression (quadratic) for better accuracy.
Does this work for polyprotic acids?
The calculator assumes a monoprotic acid. For polyprotic acids, treat each dissociation step separately using the relevant Ka value and conditions.
Can I compute pKb?
Not directly. However, once you have pKa, you can determine pKb for the conjugate base using pKa + pKb = 14 (at 25°C).
Is temperature accounted for?
No. The calculations assume standard laboratory temperature (25°C). Ka values are temperature dependent, so adjust inputs if working at different temperatures.