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+H2O⇌C5H5NH++OH− C 5 H 5 N + H 2 O ⇌ C 5 H 5 N H + + O H − The pKb p K b of pyridine is 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 acid dissociates in water:

C6H5COOH⇌C6H5COO−+H+C6H5COOH⇌C6H5COO−+H+

The pKapKa of this reaction is 4.2. In a 0.51 MM solution of benzoic acid, what percentage of the molecules are ionized?

Answer 1

1)

Given:
pKb = 8.75

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

C5H5N +H2O     ----->     C5H5NH+   +   OH-
0.32                   0         0
0.32-x                 x         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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