Pyridine is a weak base that is used in the manufacture of pesticides and plasti
ID: 939320 • 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:
C5H5N(aq)+H2O(l)C5H5NH+(aq)+OH(aq)
The pKb of pyridine is 8.75. What is the pH of a 0.455 mol L1 solution of pyridine?
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(aq)+H2O(l)C6H5COO(aq)+H3O(aq)
The pKa of this reaction is 4.2. In a 0.64 mol L1 solution of benzoic acid, what percentage of the molecules are ionized?
Explanation / Answer
a) pH of 0.455 M pyridine
pKb = -log[Kb] = 8.75
Kb = 1.78 x 10^-9
let x amount of pyridine has reacted,
Kb = x^2/0.455 = 1.78 x 10^-9
x = [OH-] = 2.84 x 10^-5 M
pOH = -log[OH-] = 4.54
pH = 14 - pOH = 9.45
b) pH of 0.64 M benzoic acid
pKa = -log[Ka] = 4.2
Ka = 6.31 x 10^-5
Ka = x^2/0.64
x = [H+] = 6.35 x 10^-3 M
pH = -log[H+] = 2.20
% of molecules ionized = (6.35 x 10^-3/0.64) x 100 = 0.99%
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