Let α be the dissociation of the weak
base,NH4OH
                      Â
NH4OH <---> NH4+ +
OH-
initial
conc.          Â
c             Â
0Â Â Â Â Â Â Â Â 0
change             Â
-cα          Â
+cα     +cα
Equb. conc.       Â
c(1-α)       cα Â
  cα
Dissociation constant, Kb = (cα x cα) / (
c(1-α)              Â
              Â
                        Â
= c α2 / (1-α)
In the case of weak bases α is very small so 1-α is taken as
1
So Kb = cα2
==> α = √ ( Kb / c )
Given Kb = 1.8x10-5
[NH4OH] = 0.10 M & [NH4+] = 0.18 M
     c(1-α) = 0.10
M    &  cα = 0.18 M
So [OH-] = cα = 0.18 M
