Kb Calculator

Calculate base dissociation constant Kb from pOH and concentration. Find degree of dissociation, pKb, and pH for weak base solutions. Find Kb for conjugate base.

🔬 Acid-Base📐 Kb = [BH⁺][OH⁻]/[B]🧪 Chemistry

Initial base concentration C (mol/L)

pOH of solution

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Kb Calculator	—	Kb = [BH⁺][OH⁻]/[B]	mol/L

Step-by-Step Examples

Example 1

Ammonia NH3

0.100 M NH3 has pOH = 2.87.

[OH-] = 10^-2.87 = 1.35x10^-3
Kb = (1.35x10^-3)^2/(0.100-1.35x10^-3) = 1.85x10^-5

✓ Kb(NH3) = 1.85x10^-5 (lit: 1.8x10^-5)

Example 2

Pyridine

0.200 M pyridine, pOH = 5.45.

[OH-] = 10^-5.45 = 3.55x10^-6
Kb = (3.55x10^-6)^2/(0.200-3.55x10^-6) ~ 6.3x10^-11/0.200 = 6.3x10^-11

✓ Kb(pyridine) = 1.7x10^-9

Example 3

From Ka

Acetic acid Ka = 1.8x10^-5. Find Kb for acetate.

Kb(CH3COO-) = Kw/Ka = 10^-14/1.8x10^-5
Kb = 5.6x10^-10

✓ Kb(acetate) = 5.6x10^-10

Real-World Applications

🧪

Buffer Preparation

Kb of weak base needed to calculate buffer pH using Henderson-Hasselbalch.

💊

Drug Bases

Many drugs are weak bases (amines). Kb determines protonation state at pH 7.4.

🌍

Nitrogen Chemistry

NH3 in soil/water. Kb determines how much NH4+ forms, affecting nitrification.

🧹

Cleaning Products

Ammonia cleaners: Kb determines pH and cleaning effectiveness.

Common Mistakes to Avoid

⚠️

Kb = Kw/Ka for conjugate acid

If you know Ka of conjugate acid (BH+), then Kb = Kw/Ka. Faster than experimental measurement.

⚠️

pOH not pH in formula

Kb formula uses [OH-], not [H+]. Calculate pOH from pH: pOH = 14 - pH. Then [OH-] = 10^-pOH.

⚠️

Strong base has no meaningful Kb

NaOH, KOH: fully dissociated, Ka/Kb concept does not apply. Kb applies only to weak bases.

Frequently Asked Questions

What is Kb? ▾

Base dissociation constant: B + H2O to BH+ + OH-. Kb = [BH+][OH-]/[B]. Larger Kb: stronger base.

What are Kb values for common bases? ▾

NaOH: strong (no Kb). NH3: 1.8x10^-5. Pyridine: 1.7x10^-9. Aniline: 4.3x10^-10. Trimethylamine: 6.3x10^-5.

How does Kb relate to Ka? ▾

Kb of base B x Ka of conjugate acid BH+ = Kw = 10^-14. pKa + pKb = 14.

What is a strong base? ▾

Fully dissociates in water: NaOH, KOH, Ca(OH)2, Ba(OH)2. [OH-] = C x n_OH (n=1 for NaOH, 2 for Ca(OH)2).

What is the relationship between structure and Kb? ▾

Electron-donating groups increase Kb (better electron donor). Electron-withdrawing groups decrease Kb. Aniline (phenyl attached) Kb = 4.3x10^-10 vs NH3 Kb = 1.8x10^-5.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

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