Ka Calculator
Calculate the acid dissociation constant from concentration and measured pH.
About Ka Calculator
For a weak monoprotic acid the dissociated proton and conjugate base concentrations are equal, so K_a reduces to [H⁺]² divided by the undissociated remainder. Percent dissociation shows how far the approximation of small dissociation holds. pK_a is the single most useful number for an acid because it equals the pH at which the acid is exactly half dissociated.
Formula
Ka
K_a = [H⁺]² ÷ (C − [H⁺]) for a monoprotic weak acid
- K_a
- Equilibrium constant for acid dissociation
- pK_a
- Negative logarithm of K_a; lower means stronger
Worked example
With the default values below, Ka Calculator returns Acid Dissociation Constant of 0.0000176102. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Initial acid concentrationminimum 0 mol/L
- 0.1 mol/L
- Measured pHaccepts -2–16
- 2.88
Results
- Acid Dissociation Constant
- 0.0000176102
- pKa
- 4.7542
- Percent Dissociation
- 1.3183%
- Hydrogen Ion Concentration
- 0.00131826 mol/L
- Acid Strength
- Weak acid
Step by step
- 1Values used
- 2Ka
- 3Acid Dissociation Constant= 0.0000176102
- 4pKa= 4.7542
- 5Percent Dissociation= 1.3183
- 6Hydrogen Ion Concentration= 0.00131826 mol/L
- 7Acid Strength= Weak acid
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 10 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Ka calculated?
K_a = [H⁺]² ÷ (C − [H⁺]) for a monoprotic weak acid For a weak monoprotic acid the dissociated proton and conjugate base concentrations are equal, so K_a reduces to [H⁺]² divided by the undissociated remainder. Percent dissociation shows how far the approximation of small dissociation holds.
Why does Ka matter?
pK_a is the single most useful number for an acid because it equals the pH at which the acid is exactly half dissociated.
What values do I need to enter?
This calculator takes 2 inputs: Initial acid concentration, Measured pH. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.