Pyridine is a weak base that is used in the manufacture of pesticides and plasti

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+H2OC5H5NH++OH

The pKb of pyridine is 8.75. What is the pH of a 0.360 M solution of pyridine?

Express the pH numerically to two decimal places.

Note:The answer is not 12.38

I got help from chegg twice and both times the answer was wrong, can somebody figure this out so i can figure out the right way to do these problems? two sig figs please.

12.38 and mastering chem said it was wrong.

7.6 is also wrong.

Explanation / Answer

Given that pKb = 8.75

C = 0.36 M

We know that

[OH-] = (Kb .C)1/2

pOH = -log [OH-]

pOH = -log(Kb .C)1/2

   = (1/2) [ -log Kb - log C ]

= (1/2) [ pKb - log C]

= (1/2) [8.75 - log 0.36]

= 4.6

pOH = 4.6

Then,

pH = 14- pH = 14- 4.6 = 9.4

Therefore,

pH = 9.4

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