List of common strong and weak acids and bases
This table sorts the acids and bases you meet most often in chemistry class into strong and weak, with their formulas. Strength describes how completely a substance ionizes in water.
Acid
| Formula | Name | |
|---|---|---|
| HCl | Hydrochloric acid | Strong |
| HBr | Hydrobromic acid | Strong |
| HI | Hydroiodic acid | Strong |
| HNO₃ | Nitric acid | Strong |
| H₂SO₄ | Sulfuric acid | Strong |
| HClO₄ | Perchloric acid | Strong |
| CH₃COOH | Acetic acid | Weak |
| HF | Hydrofluoric acid | Weak |
| H₂CO₃ | Carbonic acid | Weak |
| H₃PO₄ | Phosphoric acid | Weak |
| HCN | Hydrocyanic acid | Weak |
| C₆H₈O₇ | Citric acid | Weak |
Base
| Formula | Name | |
|---|---|---|
| NaOH | Sodium hydroxide | Strong |
| KOH | Potassium hydroxide | Strong |
| LiOH | Lithium hydroxide | Strong |
| Ca(OH)₂ | Calcium hydroxide | Strong |
| Ba(OH)₂ | Barium hydroxide | Strong |
| NH₃ | Ammonia | Weak |
| Mg(OH)₂ | Magnesium hydroxide | Weak |
| NaHCO₃ | Sodium bicarbonate | Weak |
What the table shows
The six strong acids listed are hydrochloric (HCl), hydrobromic (HBr), hydroiodic (HI), nitric (HNO₃), sulfuric (H₂SO₄) and perchloric acid (HClO₄); some textbooks add chloric acid (HClO₃) as a seventh. The strong bases are the hydroxides of group 1 metals, such as NaOH, KOH and LiOH, and of the heavier group 2 metals, such as Ca(OH)₂ and Ba(OH)₂.
Strong versus weak
A strong acid ionizes completely in water: HCl → H⁺ + Cl⁻, so 0.10 M HCl contains 0.10 M hydrogen ions. A weak acid ionizes only partly and reaches equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. In 0.10 M acetic acid only about 1.3% of the molecules ionize, so its pH is about 2.9 instead of 1.0.
Bases work the same way. Sodium hydroxide releases all of its hydroxide ions, while ammonia makes only a few by reacting with water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Tricky cases
Sulfuric acid is strong only for its first hydrogen; the second step, HSO₄⁻ ⇌ H⁺ + SO₄²⁻, is weak. Hydrofluoric acid is weak, unlike the other hydrogen halides, but it is still extremely dangerous. Calcium hydroxide is strong but only slightly soluble. Magnesium hydroxide is listed as weak because so little dissolves; some textbooks call it a strong but insoluble base instead.
Worked example: pH of a strong acid and a strong base
Find the pH of 0.010 M HCl. HCl is strong, so [H⁺] = 0.010 M and pH = −log(0.010) = 2.00. For 0.010 M NaOH, [OH⁻] = 0.010 M, so pOH = 2.00 and pH = 14.00 − 2.00 = 12.00 at 25 °C. For a weak acid you cannot assume full ionization; you need its acid dissociation constant, Ka.
FAQ
What are the strong acids?
The six common strong acids are HCl, HBr, HI, HNO₃, H₂SO₄ and HClO₄. Chloric acid, HClO₃, is sometimes counted as a seventh.
Is acetic acid a strong or weak acid?
Acetic acid, the acid in vinegar, is a weak acid. In a 0.10 M solution only about 1.3% of its molecules ionize.
Is ammonia a strong or weak base?
Ammonia is a weak base. It reacts only partly with water to form NH₄⁺ and OH⁻, so a 0.10 M solution has a pH of about 11.1 rather than 13.
What is the difference between a strong acid and a concentrated acid?
Strong describes how completely an acid ionizes; concentrated describes how much acid is dissolved per liter. A solution can be dilute but strong, such as 0.001 M HCl, or concentrated but weak, such as glacial acetic acid.