Question

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:

C5H5N+H2O⇌C5H5NH++OH−

The pKb of pyridine is 8.75. What is the pH of a 0.205 M solution of pyridine?

Express the pH numerically to two decimal places.

B)

Benzoic acid is a weak acid that has antimicrobial properties. Its sodium salt, sodium benzoate, is a preservative found in foods, medications, and personal hygiene products. Benzoic acid dissociates in water:

C6H5COOH⇌C6H5COO−+H+

The pKa of this reaction is 4.2. In a 0.56 M solution of benzoic acid, what percentage of the molecules are ionized?

Express the percentage numerically using two significant figures.

Answer 1

            C5H5N+H2O⇌C5H5NH++OH−

I            0.205                   0              0

C         -x                          +x            +x

E       0.205-x                  +x             +x

        Pkb = 8.75

      -logkb   = 8.75

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

     Kb    = [C5H5NH^+][OH^-]/[C5H5N]

    1.78*10^-9   = x*x/0.205-x

1.78*10^-9 *(0.205-x) = x^2

        x   = 1.9*10^-5

    [OH-]   = x   = 1.9*10^-5 M

POH   = -log[OH-]

             = -log1.9*10^-5

             = 4.7212

PH       = 14-POH

             = 14-4.7212

              = 9.2788

B.

                C6H5COOH⇌C6H5COO−+H+

     I         0.56                  0                  0

       C        -x                       +x                +x

        E      0.56-x                  +x               +x

            Pka   = 4.2

           -logKa   = 4.2

                 Ka   = 10^-4.2

                  Ka    = 6.3*10^-5

        Ka   =     [C6H5COO−][H+]/[C6H5COOH]

        6.3*10^-5    = x*x/(0.56-x)

       6.3*10^-5*(0.56-x) = x^2

         x    = 0.0059

   [H+]   =x = 0.0059M

       percentage ionisation    =0.0059*100/0.56    = 1.053% >>>>>answer