A weak acid is an acid that only partially ionizes (dissociates) in water. This means that in solution, only a small fraction of the acid molecules donate protons (H⁺), resulting in a lower concentration of hydrogen ions compared to strong acids. Consequently, weak acids have a moderate pH and are less acidic than strong acids.

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Characteristics of Weak Acids:

1. Partial Ionization:

   • Weak acids do not fully dissociate in water.

   • Instead, they establish an equilibrium between the undissociated acid molecules and the ions produced.

   • Example: Acetic acid 

     $\text{CH}_3\text{COOH}$
     $\text{CH}_3\text{COOH} \leftrightarrow \text{CH}_3\text{COO}^- + \text{H}^+$

2. Lower Proton Concentration:

   • Because only a fraction of the molecules dissociate, the concentration of

     H+ ions is relatively low.

3. Moderate pH:

   • Weak acids typically have pH values between 3 and 6, depending on the concentration and strength of the acid.

4. Equilibrium Constant:

   • Weak acids have a smaller acid dissociation constant (Ka​), indicating incomplete ionization.

   • Example: Ka​ for acetic acid (CH3​COOH) is approximately

     1.8×10−5

5. Examples:

   • Acetic acid ( CH3​COOH): Found in vinegar.

   • Formic acid (HCOOH)= found in ant venom

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Dissociation in Water:

The ionization of a weak acid in water is reversible and reaches equilibrium:

$\text{HA} + \text{H}_2\text{O} \leftrightarrow \text{H}_3\text{O}^+ + \text{A}^-$

 

Where:

• HA: Weak acid.

• H3​O+: Hydronium ion (proton donor in water).

• A−: Conjugate base.