1) Pyridine is a weak base that is used in the manufacture of pesticides and pla
ID: 959744 • Letter: 1
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+H2OC5H5NH++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: C6H5COOHC6H5COO+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.
Explanation / Answer
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 %
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