1.Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins....

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1.Pyridine is a weak base that is used in the manufacture ofpesticides and plastic resins. It is also a component of cigarettesmoke. Pyridine ionizes in water as follows: C5H5N+H2Oâ‡ŒC5H5NH++OHâˆ’C 5 H 5 N + H 2 O â‡Œ C 5 H 5 N H + + O H âˆ’ The pKb p K b of pyridineis 8.75. What is the pH p H of a 0.320 M M solution of pyridine?Express the pH p H 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 aciddissociates in water:
C6H5COOHâ‡ŒC6H5COOâˆ’+H+C6H5COOHâ‡ŒC6H5COOâˆ’+H+
The pKapKa of this reaction is 4.2. In a 0.51 MM solution ofbenzoic acid, what percentage of the molecules are ionized?

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3.7 Ratings (545 Votes)

Given:
pKb = 8.75

use:
pKb = -log Kb
8.75= -log Kb
Kb = 1.778*10^-9
C5H5N dissociates as:

C5H5N +H2OÂ Â Â Â ----->Â Â Â Â C5H5NH+Â Â +Â Â OH-
0.32Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â 0
0.32-xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â xÂ Â Â Â Â Â Â Â x

Kb = [C5H5NH+][OH-]/[C5H5N]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((1.778*10^-9)*0.32) = 2.385*10^-5

since c is much greater than x, our assumption is correct
so, x = 2.385*10^-5 M

So, [OH-] = x = 2.385*10^-5 M

use:
pOH = -log [OH-]
= -log (2.385*10^-5)
= 4.6224

use:
PH = 14 - pOH
= 14 - 4.6224
= 9.3776
Answer: 9.38

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