Let α be the dissociation of the weak hydrazoic acid
                          Â
HA <---> H + + A-
initial
conc.          Â
c             Â
0Â Â Â Â Â Â Â Â 0
change          Â
  Â
-cα          Â
+cα     +cα
Equb. conc.       Â
c(1-α)        cα
  cα
Dissociation constant , Ka = cα x cα / ( c(1-α)
                                       Â
= c α2 / (1-α)
In the case of weak acids α is very small so 1-α is taken as
1
So Ka = cα2
==> α = √ ( Ka / c )
Given pH = 3.21
-log[H+] = 3.21
[H+] = 10-3.21
cα = 6.16x10-4 M
         c =
concentration = 0.20 M
Plug the values we get α = 3.08 x10-3
Ka = cα2
= 0.20 x (3.08 x10-3)2
= 1.90x10-6
