Chart of Acid and Base Strength

The strength of acids and bases is determined by their ability to donate or accept protons (H⁺ ions) and can be quantified using the concepts of dissociation in water and the corresponding equilibrium constants. Understanding acid and base strength helps predict the behavior of substances in various chemical reactions, essential for fields like chemistry, biology, environmental science, and medicine.

Alfa Chemistry provides the following chart of the strengths of the most common acids and bases for you to look over. We want to help you more easily determine an acid or base's strength.

Strongest Acid Medium Acid Weakest Acid

Ka	Acid	Acid Formula				Base Formula	Base
Large	Perchloric acid	HClO4	→	H+	+	ClO4 -	Perchlorate ion
3.2 * 109	Hydroiodic acid	HI	→	H+	+	I-	Iodide
1.0 * 109	Hydrobromic acid	HBr	→	H+	+	Br-	Bromide
1.3 * 106	Hydrochloric acid	HCl	→	H+	+	Cl-	Chloride
1.0 * 103	Sulfuric acid	H2SO4	→	H+	+	HSO4 -	Hydrogen sulfate ion
2.4 * 101	Nitric acid	HNO3	→	H+	+	NO3 -	Nitrate ion
--	Hydronium ion	H3O+	→	H+	+	H2O	Water
5.4 * 10-2	Oxalic acid	HO2C2O2H	→	H+	+	HO2C2O2-	Hydrogen oxalate ion
1.3 * 10-2	Sulfurous acid	H2SO3	→	H+	+	HSO3 -	Hydrogen sulfite ion
1.0 * 10-2	Hydrogen sulfate ion	HSO4 -	→	H+	+	SO4 2-	Sulfate ion
7.1 * 10-3	Phosphoric acid	H3PO4	→	H+	+	H2PO4 -	Dihydrogen phosphate ion
7.2 * 10-4	Nitrous acid	HNO2	→	H+	+	NO3 -	Nitrite ion
6.6 * 10-4	Hydrofluoric acid	HF	→	H+	+	F-	Fluoride ion
1.8 * 10-4	Methanoic acid	HCO2H	→	H+	+	HCO2 -	Methanoate ion
6.3 * 10-5	Benzoic acid	C6H5COOH	→	H+	+	C6H5COO-	Benzoate ion
5.4 * 10-5	Hydrogen oxalate ion	HO2C2O2-	→	H+	+	O2C2O2 2-	Oxalate ion
1.8 * 10-5	Ethanoic acid	CH3COOH	→	H+	+	CH3COO	Ethanoate (acetate) ion
4.4 * 10-7	Carbonic acid	CO3 2-	→	H+	+	HCO3 -	Hydrogen carbonate ion
1.1 * 10-7	Hydrosulfuric acid	H2S	→	H+	+	HS-	Hydrogen sulfide ion
6.3 * 10-8	Dihydrogen phosphate ion	H2PO4 -	→	H+	+	HPO4 2-	Hydrogen phosphate ion
6.2 * 10-8	Hydrogen sulfite ion	HS-	→	H+	+	S2-	Sulfite ion
2.9 * 10-8	Hypochlorous acid	HClO	→	H+	+	ClO-	Hypochlorite ion
6.2 * 10-10	Hydrocyanic acid	HCN	→	H+	+	CN-	Cyanide ion
5.8 * 10-10	Ammonium ion	NH4 +	→	H+	+	NH3	Ammonia
5.8 * 10-10	Boric acid	H3BO3	→	H+	+	H2BO3 -	Dihydrogen carbonate ion
4.7 * 10-11	Hydrogen carbonate ion	HCO3 -	→	H+	+	CO3 2-	Carbonate ion
4.2 * 10-13	Hydrogen phosphate ion	HPO4 2-	→	H+	+	PO4 3-	Phosphate ion
1.8 * 10-13	Dihydrogen borate ion	H2BO3-	→	H+	+	HBO3 2-	Hydrogen borate ion
1.3 * 10-13	Hydrogen sulfide ion	HS-	→	H+	+	S 2-	Sulfide ion
1.6 * 10-14	Hydrogen borate ion	HBO3 2-	→	H+	+	BO3 3-	Borate ion
--	water	H2O	→	H+	+	OH-	Hydroxide

Weakest Base Medium Base Strongest Base

Acid Strength

Strong Acids

• Definition: Completely ionize in water.
• Examples: Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃).
• Behavior in Water: HCl → H⁺ + Cl^-

Weak Acids

• Definition: Partially ionize in water.
• Examples: Acetic acid (CH₃COOH), formic acid (HCOOH).
• Behavior in Water: CH₃COOH ⇌ H⁺ + CH₃COO^-
• Equilibrium Constant (Ka): The acid dissociation constant, Ka, measures the strength of a weak acid.

Base Strength

Strong Bases

• Definition: Completely dissociate in water.
• Examples: Sodium hydroxide (NaOH), potassium hydroxide (KOH).
• Behavior in Water: NaOH → Na⁺ + OH^-

Weak Bases

• Definition: Partially ionize in water.
• Examples: Ammonia (NH₃), methylamine (CH₃NH₂).
• Behavior in Water: NH₃ + H₂O ⇌ NH⁴+ +OH^-
• Equilibrium Constant (Kb): The base dissociation constant, Kb, measures the strength of a weak base.

Other Strength Related Knowledge

Relationship between Ka and Kb

For a conjugate acid-base pair:

Kw is the ion product of water: 1.0 × 10^-14 at 25°C.

Indicators of Strength

Strong Acids/Bases: High Ka or Kb, low pH (acids) or low pOH (bases).

Weak Acids/Bases: Low Ka or Kb, higher pH (acids) or higher pOH (bases).