Question

How to predict the relative acidity of a salt solution from the nature of the cation and anion?

Answer 1

The relative acidity of salt solution in water depends upon the nature of the salt ie.,nature of cation and anion it is made up of.

If a salt is made up of a strong acid and strong base, then its cation and anions are weak and do not react with water hence the solution remains neutral

Ex KBr (salt of KOH and HBr) remains neutral

If a salt is made up of a weak acid and strong base, then its cation is weak and anion is strong. Thus anion part of the salt reacts with cation part of water and makes the solution basic.

Ex: CH3COONa (salt of acetic acid and NaOH) forms CH3COO- and Na+

CH3COO- is strong and reacts with water to produce OH- ions in water and makes the solution relatively basic

CH3COO- + H-OH -----> CH3COOH + OH- (pH>7)

If a salt is made up of a strong acid and weak base, then its cation is strong and anion is weak. Thus cation part of the salt reacts with anion part of water and makes the solution acidic.

Ex: NH4Cl (salt of HCl and NH3) forms NH4+ and Cl-

NH4+ is strong and reacts with water to produce H+ ions in water and makes the solution relatively acidic

NH4+ + H-OH -----> NH4OH + H+ (pH<7)

If a salt is made up of a weak acid and weak base, then its cation and anions are equally strong, then solution remains neutral

Ex: CH3COONH4