pKa Calculator

Q: What is Ka?

Acid dissociation constant: Ka = [H+][A-]/[HA]. Larger Ka = stronger acid = more dissociation at equilibrium.

Q: What is the pKa range for weak acids?

Approximately pKa 0 to 14. Very weak acids: pKa > 14 (essentially no dissociation). Strong acids: pKa < 0 (complete dissociation).

Q: How does pKa relate to pH for 50% dissociation?

At pH = pKa: [HA] = [A-] (50% dissociated). Henderson-Hasselbalch: pH = pKa + log(1) = pKa.

Q: What is the effect of structure on pKa?

Electron-withdrawing groups (F, NO2, Cl) stabilize conjugate base A-, increasing Ka (decreasing pKa). Chloroacetic acid pKa=2.86 vs acetic acid pKa=4.74.

Q: What are pKa values of amino acids?

Alpha-COOH: 1.8-2.4 (carboxyl). Alpha-NH2: 8.8-10.6. Side chains vary: Asp/Glu COOH: 3.9-4.1. His imidazole: 6.0. Lys NH2: 10.5.

Calculate pKa from Ka, or find Ka from pKa. Determine if an acid is strong or weak, compare acid strengths, and find pKa from pH and concentration using Henderson-Hasselbalch.

🔬 Acid-Base📐 pKa = -log(Ka)🧪 Chemistry

Ka value (acid dissociation constant)

OR pKa value

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
pKa Calculator	—	pKa = -log(Ka)	0-14

Step-by-Step Examples

Example 1

Acetic Acid

Ka(CH3COOH) = 1.8x10^-5.

pKa = -log(1.8x10^-5) = -(log(1.8)+log(10^-5))
= -(0.255+(-5)) = 4.745

✓ pKa = 4.74 (acetic acid)

Example 2

From pKa to Ka

pKa = 9.25 (ammonium ion).

Ka = 10^(-9.25) = 5.62x10^-10 mol/L

✓ Ka = 5.62x10^-10 M

Example 3

Comparing Acids

HF: pKa=3.17. Acetic: pKa=4.74. Carbonic H2CO3: pKa=6.35. Ammonium NH4+: pKa=9.25.

Stronger acid = lower pKa
HF is stronger than acetic acid which is stronger than carbonic

✓ Order of strength: HF > acetic > carbonic > ammonium

Real-World Applications

🧪

Buffer Design

Choose buffer acid with pKa within 1 of target pH for maximum buffer capacity.

💊

Drug Design

pKa of drug determines charge state at physiological pH (7.4). Affects absorption and distribution.

🌍

Environmental Chemistry

pKa of weak acids determines their speciation in natural waters.

🧬

Biochemistry

Enzyme active sites have residues with specific pKa values (His: 6.0, Cys: 8.3, Tyr: 10.5).

Common Mistakes to Avoid

⚠️

pKa < 0 means strong acid

HCl, H2SO4, HNO3 are strong acids with pKa < 0 (essentially fully dissociated in water).

⚠️

Lower pKa = stronger acid

pKa 4.74 (acetic) vs 9.25 (ammonium): acetic is 10^(9.25-4.74) = 10^4.5 = ~32,000x stronger.

⚠️

Conjugate base pKb: pKa + pKb = 14

For conjugate acid-base pair: pKa + pKb = 14 (at 25 C). Ka x Kb = Kw = 10^-14.

Frequently Asked Questions

What is Ka? ▾

Acid dissociation constant: Ka = [H+][A-]/[HA]. Larger Ka = stronger acid = more dissociation at equilibrium.

What is the pKa range for weak acids? ▾

Approximately pKa 0 to 14. Very weak acids: pKa > 14 (essentially no dissociation). Strong acids: pKa < 0 (complete dissociation).

How does pKa relate to pH for 50% dissociation? ▾

At pH = pKa: [HA] = [A-] (50% dissociated). Henderson-Hasselbalch: pH = pKa + log(1) = pKa.

What is the effect of structure on pKa? ▾

Electron-withdrawing groups (F, NO2, Cl) stabilize conjugate base A-, increasing Ka (decreasing pKa). Chloroacetic acid pKa=2.86 vs acetic acid pKa=4.74.

What are pKa values of amino acids? ▾

Alpha-COOH: 1.8-2.4 (carboxyl). Alpha-NH2: 8.8-10.6. Side chains vary: Asp/Glu COOH: 3.9-4.1. His imidazole: 6.0. Lys NH2: 10.5.

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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