What is the pH of a 0.080 M solution of the weak base pyridine (C₅H₅N; K_b = 1.7 x 10^-9) A) 8.77 B) 5.23 C) 4.93 D) 12.90 E) 9.07

Question

Quizplus · Accepted Answer

Since pyridine is a weak base, we must use the Kb expression to calculate the pH:
Kb = [BH+][OH-]/[B]
Let x be the concentration of OH- ions. Then we have:
Kb = x^2/(0.080 - x)
1.7 x 10^-9 = x^2/(0.080 - x)
Solving for x, we get x = 2.22 x 10^-5 M.
Thus, pOH = -log(2.22 x 10^-5) = 4.65
pH + pOH = 14, so pH = 14 - 4.65 = 9.35
The closest answer choice is E) 9.07, which is the correct answer.

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