Determining the pH of a Weak Base and the Percent Ionization of a Weak Acid Unli

Question

Determining the pH of a Weak Base and the Percent Ionization of a Weak Acid Unlike strong acids and bases that ionize completely in solution, weak acids or bases partially ionize. The tendency of a weak acid or base to ionize can be quantified in several ways including K_a or K_b, pK_a or pK_b. and percent ionization*. This assumes that species of equal concentrations are being compared as percent ionization is affected by concentration. 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: C_5H_5N + H_2O C_5H_5NH^+ + OH The pKb of pyridine is 8.75. What is the pH of a 0.430 Absolution of pyridine? (Assume that the temperature is 25 degree C.)

Explanation / Answer

Part A) Pyridine

hydrolysis in water,

pyridine + H2O <==> pyridineH+ + OH-

let x amount has hydrolysed

pKb = -logKb

Kb = 1.8 x 10^-5

Kb = [pyridineH+][OH-]/[pyridine]

1.8 x 10^-5 = x^2/0.430

x = [OH-] = 2.78 x 10^-3 M

pOH = -log[OH-] = 2.556

pH = 14 - pOH = 11.44

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