Q48 of 54 Page 94

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.

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