Multiple Choice
What is the pH of a 0.400 M solution of NaF, given that the Kb of F- is 1.4 x 10^-11?
17. Acid and Base Equilibrium
pH of Weak Bases
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Calculate the pH of a solution that is 0.540 M in pyridine (C5H5N) and 0.470 M in pyridinium chloride (C5H5NHCl). The Kb for pyridine is 1.7 x 10^-9.
A
pH = 4.75
B
pH = 8.77
C
pH = 5.23
D
pH = 9.25
Verified step by step guidance1
Identify that the solution is a buffer solution, consisting of a weak base (pyridine, C5H5N) and its conjugate acid (pyridinium ion, C5H5NH+).
Use the Henderson-Hasselbalch equation for a buffer solution: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \).
Calculate \( \text{pK}_a \) from \( \text{K}_b \) using the relation \( \text{pK}_a + \text{pK}_b = 14 \). First, find \( \text{pK}_b = -\log(1.7 \times 10^{-9}) \), then \( \text{pK}_a = 14 - \text{pK}_b \).
Substitute the concentrations of the base and acid into the Henderson-Hasselbalch equation: \( [\text{Base}] = 0.540 \text{ M} \) and \( [\text{Acid}] = 0.470 \text{ M} \).
Calculate the pH using the values obtained: \( \text{pH} = \text{pK}_a + \log \left( \frac{0.540}{0.470} \right) \).
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