Question

Explain about the ionisation of weak acid and how K₂ is derived?

Answer 1

1. Weak acids are partially dissociated in water and there is an equilibrium between the undissociated acid and its dissociated ions.

2. Consider the ionisation of weak monobasic acid HA in water 

HA + H₂O ⇌ H₃O⁺ + A^- …………….(1)

3. Applying law of chemical equilibrium, the equilibrium constant K_c is given by the expression

4. In dilute solutions, water is present in large excess, hence its concentration may be taken as constant say K. Further H₃O⁺ indicates hydrated hydrogen ions, for simplicity, it may be replaced by H⁺ . So the equation (2) becomes

5. The product of two constants K and K gives another constant. Let is be K₂

The constant K_a is called dissociation constant of weak acid.