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Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins....

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 0.405 Msolution of pyridine? (Assume that the temperature is 25 ∘C.)

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 ionizes in water:

C6H5COOH⇌C6H5COO−+H+

The pKa of this reaction is 4.2. In a 0.51 M solution of benzoic acid, what percentage of the molecules are ionized?

Solutions

Expert Solution

#

Pyridine dissociate as

C5H5N + H2O C5H5NH+ + OH-

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

pyridine is monobesic base therefore

[C5H5NH+] = [OH-] = x

Kb = [x][x] /  [C5H5N]

Kb =[x]2 /  [C5H5N]

[x]2 = Kb  [C5H5N]

Kb = 10-pKb = 10-8.75 = 1.778 10-9

[x]2 = 1.778 10-9 0.405 = 7.2 10-10

[x] = 2.68 10-5

[C5H5NH+] = [OH-] = x = 2.68 10-5

pOH = -log[OH-] = -log(2.68 10-5) = 4.57

pH = 14 - pOH = 14 - 4.57 = 9.43

pH of pyridine = 9.43

#

benzoic acid dissociates as

C6H5COOH + H2O C6H5COO- + H3O+

Ka = [C6H5COO- ][H3O+] / [C6H5COOH]

but  [C6H5COO- ] = [H3O+] = x

Ka = [x][x] / [C6H5COOH]

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

Substitute the value in equation

6.3 10-5 = [x]2/ 0.51

[x]2 = 6.3 10-5 0.51 = 3.22 10-5

[x] = 0.00567 M

Concentration of H3O+ = 0.00567 M

percentage ionization = [H3O+] 100 / [C6H5COOH]

% ionization of benzoic acid = 0.00567 100 / 0.51 = 1.11 %

% ionization of benzoic acid  = 1.11%


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