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Answer to Question #25593 in Other Chemistry for Edward

Question #25593
If a buffer solution is 0.140 M in a weak acid (Ka = 1.5 × 10-5) and 0.560 M in its conjugate base, what is the pH?
Expert's answer
Since you are given Kb, the equilibrium between the weak base, water and its conjugated acid is:

B + H2O <--> BH+ + OH-

Kb = [BH+][OH-]/[B] = 1.0 X 10^-5

Substituting the concentrations of the base and its conjugate acid gives:
1.0 X 10^-5 = (0.560) [OH-] / (0.140)
[OH-] = 2.5 X 10^-6

From this, and the expression for Kw,
[H3O+] = 1.0 X 10^-14 / 2.5 X 10^-6 = 4X10^-9

and pH = -log 4X10^-9 = 8.40

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