Acid & Base Dissociation Constants (Ka/Kb) Chart
Interactive reference for Ka/Kb values. Calculate pKa, predict acid strength, and solve equilibrium problems.
Phosphoric Acid
Triprotic acid used in soft drinks and fertilizers.
Hydrofluoric Acid
Etches glass. Stronger than acetic acid but still weak.
Ethylamine
Used in organic synthesis.
Nitrous Acid
Unstable acid, distinct from the strong Nitric Acid (HNO₃).
Methylamine
Stronger base than ammonia due to electron-donating methyl group.
Formic Acid
Found in ant venom. Simplest carboxylic acid.
Acetic Acid
Vinegar component. Classic example of a weak acid.
Ammonia
Common household cleaner. Classic weak base.
Hydrazine
Rocket fuel component. Acts as a base.
Carbonic Acid
Formed when CO₂ dissolves in water. Important for blood buffering.
Dihydrogen Phosphate
Second ionization step of phosphoric acid.
Hydroxylamine
Used in organic synthesis.
Pyridine
Heterocyclic aromatic compound. Essential in organic solvents.
Hydrocyanic Acid
Extremely weak acid. Its conjugate base (Cyanide) is quite strong.
Ammonium Ion
Conjugate acid of the weak base Ammonia.
Aniline
Aromatic amine. Much weaker than aliphatic amines due to resonance.
Bicarbonate Ion
Amphoteric ion that can act as both acid and base.
Understanding Acid Strength
The strength of an acid or base is defined by its ability to dissociate (break apart) into ions in water. Strong acids dissociate completely, while weak acids only partially dissociate, creating an equilibrium state. The position of this equilibrium is quantified by the Dissociation Constant (K).
The Acid Constant (Ka)
For the reaction: HA ⇌ H⁺ + A⁻
- ✓ Large Ka = Strong Acid
- ✓ Small Ka = Weak Acid
- ✓ pKa = -log(Ka)
The Base Constant (Kb)
For the reaction: B + H₂O ⇌ BH⁺ + OH⁻
- ✓ Large Kb = Strong Base
- ✓ Small Kb = Weak Base
- ✓ pKb = -log(Kb)
The Conjugate Seesaw
There is an inverse relationship between the strength of an acid and its conjugate base. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base. This relationship is mathematically defined by the auto-ionization of water (Kw).
Fundamental Equation (at 25°C)
Why use pKa instead of Ka?
Ka values can be unwieldy (e.g., 0.000018). Converting to pKa (e.g., 4.74) condenses these numbers into a manageable scale, similar to the pH scale. A change of 1 unit in pKa represents a 10-fold change in acid strength.
Frequently Asked Questions
What is Ka in chemistry?
Ka is the acid dissociation constant, a value that measures the strength of an acid in solution. A higher Ka means the acid dissociates more fully, indicating a stronger acid.
How do you convert Ka to pKa?
The formula is pKa = -log(Ka). This logarithmic scale makes it easier to compare strengths. A lower pKa indicates a stronger acid.
What is the relationship between Ka and Kb?
For a conjugate acid-base pair at 25°C, Ka × Kb = Kw (1.0 × 10⁻¹⁴). This means as an acid gets stronger (higher Ka), its conjugate base gets weaker (lower Kb).
Why are strong acids not listed in Ka tables?
Strong acids (like HCl, HNO₃) dissociate completely in water. Their Ka values are extremely large (often > 1) and technically undefined in dilute aqueous solutions, so they are not typically listed in equilibrium tables.
What defines a "Weak" Acid?
A weak acid only partially dissociates in water, establishing an equilibrium between the acid molecule and its ions. Examples include Acetic Acid (Vinegar) and Carbonic Acid.
How do I use this table for Buffer calculations?
Use the Henderson-Hasselbalch equation: pH = pKa + log([Base]/[Acid]). Find the pKa of your weak acid from the table to plug into this formula.
What is a polyprotic acid?
Polyprotic acids can donate more than one proton. They have multiple dissociation constants (Ka₁, Ka₂, etc.). For example, Phosphoric Acid (H₃PO₄) has three dissociation steps.
Why is HF a weak acid if Fluorine is so electronegative?
Although Fluorine is electronegative, the H-F bond is extremely strong and short, making it difficult to break. This high bond energy prevents full dissociation, classifying HF as a weak acid.
Can a substance be both an acid and a base?
Yes, these are called amphoteric substances. For example, Water (H₂O) and the Bicarbonate ion (HCO₃⁻) can act as either an acid or a base depending on the conditions.
What is the pKa of water?
At 25°C, the pKa of water is often cited as 14.0 (based on Kw), but strictly speaking for the reaction H₂O ⇌ H⁺ + OH⁻, it is 15.7.