Salt Hydrolysis & Indicators
20min Part 2 / Ch4 / Lesson 5
Objectives
- Explain why salt solutions can be acidic or basic
- Understand how to choose indicators for acid-base titrations
- Read and interpret titration curves
Hydrolysis of Salts
The pH of a salt solution depends on the strength of the parent acid and base:
| Parent acid | Parent base | Salt solution | Example |
|---|---|---|---|
| Strong acid | Strong base | Neutral | NaCl |
| Strong acid | Weak base | Acidic | NH₄Cl |
| Weak acid | Strong base | Basic | CH₃COONa |
| Weak acid | Weak base | Depends on Ka, Kb | CH₃COONH₄ |
Example of hydrolysis (sodium acetate):
The conjugate base of a weak acid (CH₃COO⁻) takes H⁺ from water, leaving excess OH⁻ → basic.
Titration Curves
Choosing Indicators
Select an indicator whose color-change range includes the equivalence-point pH:
| Titration type | Equiv. pt pH | Suitable indicator |
|---|---|---|
| Strong acid + strong base | Near 7 | Methyl orange or phenolphthalein |
| Weak acid + strong base | Above 7 (basic) | Phenolphthalein (range pH 8.2–10) |
| Strong acid + weak base | Below 7 (acidic) | Methyl orange (range pH 3.1–4.4) |
| Indicator | Range | Acidic color | Basic color |
|---|---|---|---|
| Methyl orange | pH 3.1–4.4 | Red | Yellow |
| Phenolphthalein | pH 8.2–10.0 | Colorless | Pink/red |
| Litmus | pH 5.0–8.0 | Red | Blue |
Common indicators
Check Your Understanding
Q1 Is a CH₃COONa solution acidic, neutral, or basic?
Q2 Which indicator suits a weak acid titrated with strong base?
Q3 A salt of a strong acid and strong base in water is...
Exercises
Q1. Explain why NH₄Cl solution is acidic, using a chemical equation.
Solution
NH₄Cl is the salt of a strong acid (HCl) and a weak base (NH₃).
In water, NH₄⁺ hydrolyzes:
H₃O⁺ is produced, making the solution acidic.