Values of dissociation constant, ` K_(a)` are given as follows :
Acid `K_(a)`
`HCN 6.2xx10^(-10)`
`HF " " 6.6xx10^(-4)`
`HNO_(2) " " 7.2xx10^(-4)`
Correct order of increasing base strength of the base `CN^(-), F^(-) and NO_(2)^(-)` will be :

Options

A. `F^(-) lt CN^(-) lt NO_(2)^(-)`
B. `NO_(2)^(-) lt CN^(-) lt F^(-)`
C. `F^(-) lt NO_(2)^(-) lt CN^(-)`
D. `NO_(2)^(-) lt F^(-) lt CN^(-)`

Answer

C

Explanation

Higher the value of `K_(a)` lower will be the value of `pK_(a)` ie., higher will be the acidic nature. Further since `CN^(-),F^(-) and NO_(2)^(-)` are conjugate base of the acids HCN, HF and `HNO_(2)` respectively hence the correct order of base strength will be
`F^(-) lt NO_(2)^(-) lt CN^(-)`
`(because` stronger the acid weaker will be its conjugate base)