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+H2O?C5H5NH++OH?

1. The pKb of pyridine is 8.75. What is the pH of a 0.490M  solution of pyridine?

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+

2. The pKa of this reaction is 4.2. In a 0.69M  solution of benzoic acid, what percentage of the molecules are ionized?

Explanation / Answer

1- sol-

Kb = 10^-8.75 = 2 x 10^-9

2 x 10^-9 = x^2 / 0.490-x

x = [OH-]= 0.0000313 M

pOH = -Log[OH-] = Log(3.13x10^-5) = -4.50

pOH = 4.50

because, pH + pOH = 14.00, so pH = 14.00 - 4.50 = 9.50

so, pH = 9.50

2- sol-

Ka = 10^-4.2 =6.3 x 10^-5 M

6.3 x 10^-5 = x^2 / 0.69-x

x = 0.00656

0.00656 x 100 / 0.69 = 0.95 %

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