Part A Pyridine is a weak base that is used in the manufacture of pesticides and

Question

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:

C5H5N+H2O?C5H5NH++OH?

The pKb of pyridine is 8.75. What is the pH of a 0.335M solution of pyridine?

Part 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 0.56M solution of benzoic acid, what percentage of the molecules are ionized

Explanation / Answer

Part A

Given reaction :

             C5H5N + H2O ---> C5H5NH+ + OH-

Initial        0.335                      0               0

Equil.      0.335-x                     x              x

Equilibrium constant, Kb = x2/(0.335-x)

Given, pKb = 8.75, so Kb = 10-8.75 = 1.77*10-9

x2/(0.335-x) = 1.77*10-9

Solving for +ve value of x , we get :

x = 2.43*10-5

Thus, [OH-] = x = 2.43*10-5

So, pOH = 4.61

Thus, pH = 14-pOH = 9.39

Part B

Given reaction :

            C6H5COOH ----> C6H5COO- + H+

Initial           0.56                     0           0

Equil.          0.56-x                  x            x

For this reaction,

Ka = 10-pKa = 10-4.2 = 6.3*10-5

Ka = 6.3*10-5 = x2/(0.56-x)

SOlving for +ve value of x, we get :
x = 0.59*10-2

Thus, % of molecules ionized = Molecules ionized/Total molecules*100

= (x/0.56)*100) = 1.053%

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