Pyridine is a weak base that is used in the manufacture of pesticides and plasti
ID: 811833 • 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+H2O?C5H5NH++OH?
The pKb of pyridine is 8.75. What is the pH of a 0.140M solution of pyridine?
Express the pH numerically to two decimal places.
Part B
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 0.72M solution of benzoic acid, what percentage of the molecules are ionized?
Express your answer to two significant figures and include the appropriate units.
percent ionized=
Explanation / Answer
C5H5N+H2O<-----------------> C5H5NH++ OH-
0.14 0 0
0.14-x x x
Kb = [C5H5NH+][ OH- ]/[C5H5N]
1.778 x10^-9 =x^2/0.14-x
x= 1.577 x10^-5
[OH-]= x= 1.577 x10^-5
[H+]= 0.634 x 10^-9
pH= -log[H+]
= -log(0.634 x 10^-9)
=8.80
part B
C6H5COOH<--------------->C6H5COO- + H+
0.72 0 0
0.72-x x x
Ka= [C6H5COO-][ H+]/[C6H5COOH]
63 x10^-6 = x2/0.72-x
x = sqrt (ka/C)
= 0.0093
= 0.93% ionised
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