How to deduce Nature of Salt in Salt Hydrolysis

A salt can be acidic, neutral or alkaline.

We can deduce the nature of the salt from its constituent ions, namely:

1. ion formed from a weak acid will be a conjugate base, eg CH3COO- (from weak acid CH3COOH)

2. ion formed from a strong acid will be neutral, eg Cl- (from strong acid HCl)

3. ion formed from a weak base will be a conjugate acid, eg NH4+ (from weak base NH3)

4. ion formed from a strong base will be neutral, eg Na+ (from strong base NaOH)

This can be summarised as shown:

salt hydrolysis nature of ions summary

Next we can move on to deduce the nature of a salt.

Let's have some examples.

1. Sodium chloride NaCl is neutral
What we do is simply just deduce the nature of each constituent ion based on the acid or base that forms that ion.

salt hydrolysis sodium chloride neutral

Na+ is neutral since it is formed from strong base NaOH

Cl- is neutral since it is formed from strong acid HCl

Since both cation and anion are neutral, NaCl must be neutral.

2. Sodium ethanoate CH3COONa is alkaline
salt hydrolysis sodium ethanoate is alkaline

Na+ is neutral since it is formed from strong base NaOH.

CH3COO- is alkaline since it is the conjugate base of weak acid CH3COOH.

Since we have a conjugate base, CH3COO- will dissociate in water to give OH-, hence the salt is alkaline.

3. Ammonium chloride NH4Cl is acidic
salt hydrolysis ammonium chloride is acidic

Cl- is neutral since it is formed from strong acid HCl.

NH4+ is acidic since it is the conjugate acid of weak base NH3.

Since we have a conjugate acid, NH4+ will dissociate in water to give H+, hence the salt is acidic.

Watch this video to learn an easy way to understand salt hydrolysis, a concept that students often find confusing in Ionic Equilibria!

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

You can also view this video lesson with screenshots and detailed explanation

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