Determining the Strength of Acids and Bases

Strong electrolytes are completely dissociated into ions in water. The acid or base molecule does not exist in aqueous solution, only ions. Weak electrolytes are incompletely dissociated.

Strong Acids

Strong acids completely dissociate in water, forming H⁺ and an anion. There are six strong acids. The others are considered to be weak acids. You should commit the strong acids to memory:

HCl - hydrochloric acid
HNO₃ - nitric acid
H₂SO₄ - sulfuric acid
HBr - hydrobromic acid
HI - hydroiodic acid
HClO₄ - perchloric acid

If the acid is 100% dissociated in solutions of 1.0 M or less, it is called strong. Sulfuric acid is considered strong only in its first dissociation step; 100% dissociation isn't true as solutions become more concentrated.

H₂SO₄ → H⁺ + HSO₄^-

Weak Acids

A weak acid only partially dissociates in water to give H⁺ and the anion. Examples of weak acids include hydrofluoric acid, HF, and acetic acid, CH₃COOH. Weak acids include:

Molecules that contain an ionizable proton. A molecule with a formula starting with H usually is an acid.
Organic acids containing one or more carboxyl group, -COOH. The H is ionizable.
Anions with an ionizable proton. (e.g., HSO₄^- → H⁺ + SO₄^2-)
Cations
- Transition metal cations
- Heavy metal cations with high charge
- NH₄⁺ dissociates into NH₃ + H⁺

Strong Bases

Strong bases dissociate 100% into the cation and OH^- (hydroxide ion). The hydroxides of the Group I and Group II metals usually are considered to be strong bases.

LiOH - lithium hydroxide
NaOH - sodium hydroxide
KOH - potassium hydroxide
RbOH - rubidium hydroxide
CsOH - cesium hydroxide
*Ca(OH)₂ - calcium hydroxide
*Sr(OH)₂ - strontium hydroxide
*Ba(OH)₂ - barium hydroxide

* These bases completely dissociate in solutions of 0.01 M or less. The other bases make solutions of 1.0 M and are 100% dissociated at that concentration. There are other strong bases than those listed, but they are not often encountered.

Weak Bases

Examples of weak bases include ammonia, NH₃, and diethylamine, (CH₃CH₂)₂NH. Like weak acids, weak bases do not completely dissociate in aqueous solution.