Pyridine is aweak base that is used in the manufacture of pesticides and plastic

Question

Pyridine is aweak base that is used in the manufacture of pesticides and plasticresins. It is also a component of cigarette smoke. Pyridine ionizesin water as follows:

           C5H5N + H2O left and right arrows C5H5NH+ +OH-

The pKb ofpyridine is 8.75. What is the pH of a 0.145 M solutionof pyridine?

Part B

Benzoic acid isa weak acid that has antimicrobial properties. Its sodium salt,sodium benzoate, is a preservative found in foods, medications, andpersonal hygiene products. Benzoic acid dissociates in water:

          C6H5COOH left and right arrows C6H5COO- + H+

            

The pKa of thisreaction is 4.2. In a 0.50 M solution of benzoic acid, whatpercentage of the molecules are ionized?

Explanation / Answer

The second one is based on the equation % Dissociation =[conjugate acid] / [acid] . Since you know pka = 4.2 and theequation is pka = -logKa you can find Ka. Now use theequation Ka = [conjugate acid][conjugate base] / [acid] and sinceyou know concentration of acid and the fact that concentration ofconjugate acid and conjugate base are equal, Ka = x^2 / 0.5. Now that you know concentration of the conjugate acid and theacid, you can find % Dissociation using the originalequation.

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