Question
Predict the pH of salt solutions based on the strength of the parent acid and base. Derive the pH formula for a salt of a weak acid and strong base.
(JEE Main, NEET, CBSE 11 — salt hydrolysis pH calculation is a high-frequency numerical)
Solution — Step by Step
| Salt Type | Parent Acid | Parent Base | Solution pH | Example |
|---|---|---|---|---|
| Strong acid + Strong base | Strong | Strong | Neutral (pH = 7) | NaCl, KNO3 |
| Weak acid + Strong base | Weak | Strong | Basic (pH > 7) | CH3COONa, Na2CO3 |
| Strong acid + Weak base | Strong | Weak | Acidic (pH < 7) | NH4Cl, FeCl3 |
| Weak acid + Weak base | Weak | Weak | Depends on Ka vs Kb | CH3COONH4 |
The rule: the “stronger parent wins.” If the base is stronger (weak acid + strong base), the solution is basic.
For a salt like CH3COONa (concentration ):
The acetate ion hydrolyses:
Or equivalently: … wait, let us write it correctly:
For a salt like NH4Cl (concentration ):
For weak acid + weak base salt (like CH3COONH4):
Notice: the pH of a weak acid + weak base salt is independent of concentration.
graph TD
A["Salt Solution"] --> B{"Parent acid and base?"}
B -->|"Strong + Strong"| C["pH = 7 Neutral"]
B -->|"Weak acid + Strong base"| D["pH > 7 Basic"]
B -->|"Strong acid + Weak base"| E["pH < 7 Acidic"]
B -->|"Weak + Weak"| F{"Compare Ka and Kb"}
F -->|"Ka > Kb"| G["Acidic"]
F -->|"Ka < Kb"| H["Basic"]
F -->|"Ka = Kb"| I["Neutral"]Why This Works
When a salt dissolves, it dissociates completely. If one of the ions comes from a weak parent (weak acid or weak base), that ion reacts with water (hydrolyses) to partially regenerate the weak parent. This reaction produces either OH- (making the solution basic) or H+ (making it acidic).
For CH3COONa: the Na+ ion does not hydrolyse (comes from strong base). But CH3COO- hydrolyses — it grabs a proton from water, producing CH3COOH and OH-. The excess OH- makes the solution basic.
Alternative Method
For JEE numericals, the quick formula approach:
- Weak acid + strong base:
- Strong acid + weak base:
- Weak + weak:
Just plug in values. Remember if you need to convert.
Common Mistake
The most common error: saying “NaCl solution is always neutral.” NaCl in pure water gives pH 7. But NaCl in a buffer or in the presence of other solutes may not be neutral. The neutrality applies only to the salt’s own hydrolysis behaviour.
Also, students forget that the pH of a weak acid + weak base salt does not depend on concentration. The formula has no concentration term. Diluting CH3COONH4 does not change its pH significantly. This is a common JEE trap.