Pyridine is a weak base that is used in the manufacture of pesticides and plasti
ID: 1048220 • Letter: P
Question
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: C_5H_5N + H_2O doubleheadarrow C_5H_5 NH^+ + OH^- The pK_b of pyridine is 8.75. What is the pH of a 0.415 Msolution of pyridine? (Assume that the temperature is 25 degree C.) Express the pH numerically to two decimal places. 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: C_6H_5 COOH doubleheadarrow C_6H_5 COO^- =H^+ The pKa of this reaction is 4.2. In a 0.72 M solution of benzoic acid, what percentage of the molecules are ionized? Express your answer to two significant figures and include the appropriate units.Explanation / Answer
pKb of pyridine = 8.75
C5H5N + H2O ------------> C5H5NH+ + OH-
0.415 0 0
0.415 - x x x
Kb = x^2 / 0.415 - x
1.778 x 10^-9 = x^2 / 0.415 - x
x = 2.716 x 10^-5 M
[OH-] = 2.716 x 10^-5 M
pOH = -log[OH-] = -log (2.716 x 10^-5 M)
= 4.57
pH = 9.43
Part B )
Ka = 10^-pKa = 10^-4.2 = 6.31 x 10^-5
[H+] = sqrt (Ka x C)
= sqrt (6.31 x 10^-5 x 0.72)
= 6.74 x 10^-3 M
% ionization = ([H+] / C ) x 100
= (6.31 x 10^-5 / 0.72 ) x 100
% ionization = 0.94
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