H3PO4 (phosphoric acid) is a weak acid.
Understanding Acid Strength
The strength of an acid refers to its ability to donate protons (H+) when dissolved in water. Strong acids completely dissociate into ions, while weak acids only partially dissociate. A helpful approach is to memorize the strong acids; anything else is generally considered weak.
Phosphoric Acid's Behavior
When H3PO4 dissolves in water, it undergoes ionization. However, unlike strong acids, this ionization is not complete. This means that only a fraction of the H3PO4 molecules donate their protons, resulting in a lower concentration of H+ ions in the solution compared to a strong acid of the same concentration.
Strong vs. Weak Acids
| Feature | Strong Acid | Weak Acid |
|---|---|---|
| Dissociation | Complete | Partial |
| H+ Concentration | High | Low |
| Examples | HCl, H2SO4, HNO3, HBr, HI, HClO4, HClO3 | H3PO4, CH3COOH (acetic acid), HF |
Memorizing the seven common strong acids (Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid (HNO3), Hydrobromic acid (HBr), Hydroiodic acid (HI), Perchloric acid (HClO4), and Chloric acid (HClO3)) makes identifying weak acids like H3PO4 easier.
Conclusion
Because phosphoric acid (H3PO4) only partially dissociates in water, it is classified as a weak acid.
