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Pyridine, C5H5N, is a commonly used reagent in the lab. It is a highly flammable, weakly basic, water-miscible liquid with a distinctive, unpleasant fish-like smell. Suppose now in a buffer solution, the concentration of [C5H5N] is 0.01 M and the concentration of [C5H6N+] (the conjugate acid) is 0.02M. What pH is the buffer? (Given that the Kb for C5H5N is 1.8 x 10−9 )

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Answer:

5.0

Explanation:

We have a buffer system formed by a weak base (C₅H₅N) and its conjugate acid (C₅H₆N⁺). We can calculate the pOH using the Henderson-Hasselbach's equation.

pOH = pKb + log [acid]/[base]

pOH = -log 1.8 × 10⁻⁹ + log 0.02/0.01

pOH = 9.0

Then, we will calculate the pH.

pH + pOH = 14

pH = 14 - pOH = 14 - 9.0 = 5.0

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