Note: In the following questions a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.
Assertion (A) : Aqueous solution of ammonium carbonate is basic.
Reason (R) : Acidic/basic nature of a salt solution of a salt of weak acid and weak base depends on Ka and Kb value of the acid and the base forming it.
Ammonium carbonate is a salt of weak acid and weak base.
H2CO3 + NH4OH � (NH)2CO3
Weak acid weak base
pH = 
[
+ 
- 
]
Where, 
= -log (Kw)
Kw (ionic product of water)= 10-14
= -log (10-14) = 14
On comparing Ka and Kb of H2CO3 and NH4OH,
Ka < Kb
On taking –log on both sides,
-log Ka > -log Kb
pKa > pKb
So, pKa - pKb > 0
Therefore, pH > 
[
i.e. pH > 7 which indicates basic solution.
From this it is clearly evident that acidic/basic nature of a solution of a salt of weak acid and weak base depends on Ka and Kb value of the acid and the base forming it.