Question

In: Chemistry

Calculate the pH of a 0.0333 M aqueous solution of dimethylamine ((CH3)2NH, Kb = 5.9×10-4) and...

Calculate the pH of a 0.0333 M aqueous solution of dimethylamine ((CH3)2NH, Kb = 5.9×10-4) and the equilibrium concentrations of the weak base and its conjugate acid.

pH =
[(CH3)2NH]equilibrium = M
[(CH3)2NH2+ ]equilibrium = M

Solutions

Expert Solution

                  ((CH3)2NH + H2O--------> [(CH3)2NH2+ + OH-

     I             0.0333                               0                     0

    C             -x                                      +x                     +x

   E            0.0333-x                               +x                     +x

         Kb   = [(CH3)2NH2+] [OH-]/[(CH3)2NH]

       5.9*10-4     = x*x/0.0333-x

      5.9*10-4 *(0.0333-x) = x^2

       x= 0.004147

     [OH-] = x = 0.004147M

[(CH3)2NH] = 0.0333-x     = 0.0333-0.004147 = 0.0292M

[(CH3)2NH2+ ]   = x= 0.004147M

   POH   = -log[OH-]

             = -log0.004147   = 2.3823

PH    = 14-POH

           = 14-2.3823 = 11.6177


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