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

Question

Part A 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: The pKb of pyridine is 8.75. What is the pH of a 0.235 M solution of pyridine? (Assume that the temperature is 25 °C.) 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: C&H;, COOH CH, COO+H+ The pKa of this reaction is 4.2. In a 0.57 M solution of benzoic acid, what percentage of the molecules are ionized?

Explanation / Answer

pOH = 1/2(pKb -log(c_base))
pOH = 1/2(8.75 - log(0.235))
pO = 1/2*9.379
pOH = 4.69

pH = 14 - 4.69
pH = 9.31

pH = 1/2(pKa - log(c_acid))
pH = 1/2(4.2 - log(0.57)
pH = 1/2(4.44)
pH = 2.22

= c_H3O+/c_acid
= 10^-2.22/0.57
= 0.0106= 1.06%

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