Multiple Choice
What is the pH of a 0.200 M CH3NH3Br solution, given that the Kb of CH3NH2 is 4.4 × 10⁻⁴?
17. Acid and Base Equilibrium
pH of Weak Acids
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What is the pH of a solution made by adding 0.30 mol of acetic acid and 0.30 mol of sodium acetate to enough water to make 1.0 L of solution? (Ka = 1.8 × 10⁻⁵)
A
4.74
B
2.87
C
7.00
D
5.00
Verified step by step guidance1
Identify that the solution is a buffer solution, consisting of acetic acid (CH₃COOH) and its conjugate base, sodium acetate (CH₃COONa).
Use the Henderson-Hasselbalch equation to calculate the pH of the buffer solution: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where [A⁻] is the concentration of the acetate ion and [HA] is the concentration of acetic acid.
Calculate the pKa from the given Ka value: \( \text{pKa} = -\log(1.8 \times 10^{-5}) \).
Since the concentrations of acetic acid and acetate ion are equal (0.30 M each), the ratio \( \frac{[\text{A}^-]}{[\text{HA}]} \) is 1, and \( \log(1) = 0 \).
Substitute the values into the Henderson-Hasselbalch equation to find the pH: \( \text{pH} = \text{pKa} + 0 \).
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