Multiple Choice
Calculate the pH of a solution that is 0.0190 M in NaOH and 0.0870 M in sodium acetate (K_a for CH_3COOH = 1.75 x 10^-5). What is the pH?
17. Acid and Base Equilibrium
pH of Weak Bases
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Calculate the pH of a solution that is 0.205 M in CH3NH2 and 1.55 M in CH3NH3Br. Assume the Kb for CH3NH2 is 4.4 x 10^-4.
A
pH = 9.82
B
pH = 8.45
C
pH = 10.18
D
pH = 11.22
Verified step by step guidance1
Identify the components of the solution: CH3NH2 is a weak base and CH3NH3Br is its conjugate acid. This is a buffer solution.
Use the Henderson-Hasselbalch equation for a buffer solution: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \).
Calculate \( \text{pK}_a \) using the relation \( \text{pK}_a = 14 - \text{pK}_b \). First, find \( \text{pK}_b \) from \( \text{K}_b \) using \( \text{pK}_b = -\log(\text{K}_b) \).
Substitute the concentrations into the Henderson-Hasselbalch equation: \( [\text{Base}] = 0.205 \text{ M} \) and \( [\text{Acid}] = 1.55 \text{ M} \).
Calculate the pH using the values obtained for \( \text{pK}_a \) and the concentrations of the base and acid.
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