Question

1) 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 0.310 M solution of pyridine? Express the pH numerically to two decimal places

2)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 0.61 M solution of benzoic acid, what percentage of the molecules are ionized? Express your answer to two significant figures and include the appropriate units.

Answer 1

Kb of pyridine = 10^-pKb = 1.78 x 10^-9

C5H5N + H2O <--------------------> C5H5NH+ + OH−

0.310                                                   0               0

0.310 - x                                              x               x

Kb = x^2 / 0.310 - x

1.78 x 10^-9 = x^2 / 0.310 - x

x = 2.35 x 10^-5

[OH-] = 2.35 x 10^-5 M

pOH = -log[OH-] = -log (2.35 x 10^-5 ) = 4.63

pH = 9.37

2)

Ka = 6.31 x 10^-5

C = 0.61

[H+] = sqrt (Ka x C)

        = sqrt (6.31 x 10^-5 x 0.61)

         = 6.2 x 10^-3 M

% ionized = ([H+] / C ) x 100

                = (6.2 x 10^-3 / 0.61 ) x 100

                = 1.0 %