(C2H5)2NH2Br it is a
salt of strong acid and weak base. so cation participate in the
reaction
(C2H5)2NH2+
+ H2OÂ Â Â --------------------->
(C2H5)2NH2 + H3O+
  Â
0.51Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0
0.51 -
x                                                          Â
x                 Â
x
Ka = Kw / Kb = 1.0 x 10^-14 / 6.9 x 10-4. = 1.45 x
10^-11
Ka = [ (C2H5)2NH2
][H3O+] /
[(C2H5)2NH2+
]
1.45 x 10^-11 = x^2 / 0.51 - x
x = 2.71 x 10^-6
[H3O+] = x = 2.71 x 10^-6 M
pH = -log[H3O+]
     = -log (2.71 x 10^-6)
     = 5.57
pH = 5.57
  Â
