Which of the following anions has the largest base dissociation constant NO2- , PO4^3- , N3^-1 ?

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Explanation: 
NO2  anion has the
Ka = 4.5 x 10−4 
 
so, using
Kw = Ka.Kb
Kb = 2.2 x 10−11
 
Similary find the Kb for  PO4^3- , N3-
 
PO43− Ka  = 4.2 x 10−13 , so Kb = 2.4 x 10−2
 
N3 Ka = 1.9 x 10−5 , so Kb = 5.3 x 10−10
 
Clearly  PO43− has the largest base-dissociation constant .
 
 
 
Answer: 

PO43−