Calculator Inputs
Example Data Table
| Acid | Ka | C₀ (M) | [A⁻]₀ (M) | Approx. pH | Approx. % Dissociation |
|---|---|---|---|---|---|
| Acetic Acid | 1.8E-5 | 0.100 | 0.000 | 2.8753 | 1.3327 |
| Acetic Acid | 1.8E-5 | 0.010 | 0.000 | 3.3816 | 4.1536 |
| Acetic Acid | 1.8E-5 | 0.100 | 0.050 | 4.4442 | 0.0360 |
| Carbonic Acid | 6.3E-8 | 0.200 | 0.000 | 3.9499 | 0.0561 |
These rows show typical weak-acid behavior and the suppressing effect of an added conjugate base.
Formula Used
Reaction: HA ⇌ H+ + A-
Equilibrium expression: Ka = [H+][A-] / [HA]
With initial acid concentration C₀ and initial conjugate-base concentration S:
Ka = x(S + x) / (C₀ - x)
x² + x(S + Ka) - KaC₀ = 0
x = (-(S + Ka) + √((S + Ka)² + 4KaC₀)) / 2
Then [H⁺] = x, [A⁻] = S + x, [HA] = C₀ - x.
pH = -log10([H⁺]) and % dissociation = (x / C₀) × 100.
How to Use This Calculator
- Enter the acid name for reference in your report.
- Select whether you want to provide Ka or pKa.
- Enter the equilibrium constant value in the chosen mode.
- Type the initial weak-acid concentration, C₀, in mol/L.
- Optionally add an initial conjugate-base concentration to model a common-ion effect.
- Choose display precision and the number of graph sample points.
- Press Calculate Dissociation to show the results above the form.
- Download a CSV or PDF report if you need a saved copy.
FAQs
1) What does this acid dissociation calculator solve?
It calculates equilibrium hydrogen-ion concentration, pH, pOH, conjugate-base concentration, undissociated acid concentration, and percent dissociation for a monoprotic weak acid system.
2) Can I use pKa instead of Ka?
Yes. Switch the input mode to pKa, enter the value, and the page converts it internally to Ka before solving the equilibrium equation.
3) Why does adding conjugate base lower dissociation?
An added conjugate base shifts equilibrium toward the undissociated acid. This common-ion effect lowers hydrogen-ion production and raises the final pH.
4) Is this suitable for strong acids?
No. Strong acids dissociate almost completely, so weak-acid equilibrium treatment is unnecessary. This page is designed for weak monoprotic acids only.
5) What assumptions are built into the calculation?
The model assumes one dissociating proton, ideal-solution behavior, and standard water ionization at 25°C. Activity corrections and ionic-strength corrections are not included.
6) Why does the graph use many concentrations?
The graph helps you compare how pH and dissociation change as initial acid concentration moves above or below your chosen value. It is useful for sensitivity analysis.
7) Why do shortcut and exact results differ?
Shortcut formulas ignore some terms to simplify the algebra. When dissociation is not very small, the exact quadratic result is more reliable.
8) Which units should I enter?
Enter concentrations in mol/L. Ka is unitless in the displayed form, pKa is logarithmic, and pH and pOH are dimensionless values.