Part A Pyridine is a weak base that is used in the manufactu

Part A Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins. It is also a component of cigarette smoke. Pyridine ionizes in water as follows: The pKb of pyridine is 8.75. What is the pH of a 0.125 Msolution of pyridine? Express the pH numerically to two decimal places. View Available Hint(s) pH-

Solution

     Pkb = 8.75

    -logKb   = 8.75

         Kb   = 10^-8.75   = 1.78*10^-9

C5H5N + H2O ---------> C5H5NH^+ (aq) + OH^- (aq)

I                 0.125                               0                       0

C               -x                                    +x                       +x

E             0.125-x                               +x                      +x

              Kb   = [C5H5NH^+][OH^-]/[C5H5N]

            1.78*10^-9 = x*x/0.125-x

            1.78*10^-9 *(0.125-x) = x^2

             x   = 1.49*10^-5

            [OH^-]     =x = 1.49*10^-5M

              POH   = -log[OH^-]

                         = -log1.49*10^-5

                         = 4.8268

           PH     = 14-POH

                      = 14-4.8268

                       = 9.1732

part-B

PKa = 4.2

-logKa = 4.2

    Ka   = 10^-4.2

   Ka    = 6.3*10^-5

            C6H5COOH --------------> C6H5COO^- (aq) + H^+ (aq)

   I            0.8                                   0                          0

   C          -x                                     +x                         +x

    E         0.8-x                                 +x                        +x

                    Ka   = [C6H5COO^-][H^+]/[C6H5COOH]

                    6.3*10^-5   =    x*x/0.8-x

                   6.3*10^-5 *(0.8-x) = x^2

                   x = 0.007

             [H^+]   = x = 0.007M

            percent ionisation = final conc*100/initial conc

                                          = 0.007*100/0.8   = 0.875%

I