Question
Values of dissociation constant, $${K_a}$$ are given as follows :
$$\eqalign{
& {\text{Acid}}\,\,\,\,\,\,\,{K_a} \cr
& HCN\,\,\,\,\,6.2 \times {10^{ - 10}} \cr
& HF\,\,\,\,\,\,\,\,\,\,7.2 \times {10^{ - 4}} \cr
& HN{O_2}\,\,\,4.0 \times {10^{ - 4}} \cr} $$
Correct order of increasing base strength of the base $$C{N^ - },{F^ - }$$ and $$NO_2^ - $$ will be :
A.
$${F^ - } < C{H^ - } < NO_2^ - $$
B.
$$NO_2^ - < C{N^ - } < {F^ - }$$
C.
$${F^ - } < NO_2^ - < C{N^ - }$$
D.
$$NO_2^ - < {F^ - } < C{N^ - }$$
Answer :
$${F^ - } < NO_2^ - < C{N^ - }$$
Solution :
Higher the value of $${K_a}$$ lower will be the value of $$p{K_a}$$ i.e. higher will be the acidic nature. Further $$C{N^ - },{F^ - }$$ and $$NO_2^ - $$ are conjugate base of the acids $$HCN,HF$$ and $$HN{O_2}$$ respectively hence the correct order of base strength will be $${F^ - } < NO_2^ - < C{N^ - }$$
( $$\because $$ stronger the acid weaker will be its conjugate base )