Ka (aprox α²·C)
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
The Ka (approx α²·C) calculator estimates the acid dissociation constant (Ka) for weak acids using the degree of ionization (α) and initial concentration (C). It applies the formula Ka ≈ α²·C/(1−α), a simplification valid when α is small (α << 1). This tool is practical for analytical chemistry to predict ionic behavior in dilute solutions.
The formula works best for moderate concentrations of weak acids with low ionization (α < 0.1). If α exceeds 0.1, the approximation may underestimate Ka, requiring exact methods like the quadratic solution. To use: input α and C, then click 'Calculate'.
Key precautions: ensure α is in decimal form (not percentage) and C is in mol/L. Unreasonable results (e.g., negative values) indicate incorrect inputs. For very low concentrations or extremely weak acids, consider alternative formulas.
Use cases: lab pH analysis, chemical equilibrium studies, or pre-lab calculations. Avoid with strong acids, which fully dissociate, invalidating the approximation.
Frequently asked questions
When to use this approximation instead of the exact formula?
Use it when the degree of ionization (α) is less than 0.1. For α > 0.1, apply the exact formula Ka = α²·C / (1−α).
Why might the result exceed the input concentration?
This occurs if α > 1, which is chemically impossible. Check your input values.
Can I use this calculator for weak bases?
Not directly. For weak bases, calculate Kb using Kb = Kw / Ka, where Kw is water's ionization constant.
What does a negative result mean?
It indicates invalid inputs. Ensure α is between 0 and 1 and C is positive.