A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains.

$\backslash mathrm\{\; HA\_\{(aq)\}\; \backslash ,\; \backslash leftrightarrow\; \backslash ,\; H^+\backslash ,\_\{(aq)\}\; +\backslash ,\; A^-\backslash ,\_\{(aq)\}\; \}$

The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (K_a):

$\backslash mathrm\{\; K\_a\backslash ,\; =\backslash ,\; \backslash frac\; \{**\}\{*\}\; \}$

The greater the value of K_a, the more the formation of H⁺ is favored, and the lower the pH of the solution. The K_a of weak acids varies between 1.8×10^-16 and 55.5. Acids with a K_a less than 1.8×10^-16 are weaker acids than water. Acids with a K_a of greater than 55.5 are strong acids and almost totally dissociate when dissolved in water.

The vast majority of acids are weak acids.

Weak Acid Ionisers

• Phosphoric acid, H₃PO₄
• Ethanoic acid, CH₃COOH
• Hydrofluoric acid, HF
• Acetylsalicylic acid, C₆H₄OCOCH₃CO₂H
• Nicotinic acid, C₅H₅NCOOH
• Pyruvic acid, CH₃COCOOH
• Bromothymol blue, C₂₁H₁₅NaBr₂O₃S

Acids

Acide faible | Zwak zuur | Heikko happo | Svag syra