Kb Calculator
Calculate the base dissociation constant from concentration and measured pH.
About Kb Calculator
K_b mirrors K_a using hydroxide rather than hydrogen ion concentration. The conjugate relationship means knowing one member of an acid-base pair fixes the other completely. Tabulated data usually gives pK_a even for bases, so the conjugate conversion is needed constantly in practice.
Formula
Kb
K_b = [OH⁻]² ÷ (C − [OH⁻]); pK_a + pK_b = 14 for a conjugate pair
- K_b
- Equilibrium constant for base protonation
- conjugate relationship
- pK_a of the conjugate acid is 14 minus pK_b
Worked example
With the default values below, Kb Calculator returns Base Dissociation Constant of 0.0000184458. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Initial base concentrationminimum 0 mol/L
- 0.1 mol/L
- Measured pHaccepts -2–16
- 11.13
Results
- Base Dissociation Constant
- 0.0000184458
- pKb
- 4.7341
- Percent Dissociation
- 1.3490%
- Hydroxide Ion Concentration
- 0.00134896 mol/L
- pKa of the Conjugate Acid
- 9.2659
Step by step
- 1Values used
- 2Kb
- 3Base Dissociation Constant= 0.0000184458
- 4pKb= 4.7341
- 5Percent Dissociation= 1.3490
- 6Hydroxide Ion Concentration= 0.00134896 mol/L
- 7pKa of the Conjugate Acid= 9.2659
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 Kb calculated?
K_b = [OH⁻]² ÷ (C − [OH⁻]); pK_a + pK_b = 14 for a conjugate pair K_b mirrors K_a using hydroxide rather than hydrogen ion concentration. The conjugate relationship means knowing one member of an acid-base pair fixes the other completely.
Why does Kb matter?
Tabulated data usually gives pK_a even for bases, so the conjugate conversion is needed constantly in practice.
What values do I need to enter?
This calculator takes 2 inputs: Initial base 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.