Multiple Choice
Calculate the pH of a 0.03 M aqueous solution of a weak acid, HA, that has a pKa of 3.39.
17. Acid and Base Equilibrium
pH of Weak Acids
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Calculate the pH of a buffer that contains 1.5 M CH3COOH and 1.3 M NaCH3COO. Given that the acid dissociation constant (Ka) for CH3COOH is 1.8 x 10^-5.
A
3.50
B
4.74
C
5.00
D
6.00
Verified step by step guidance1
Identify the components of the buffer solution: CH3COOH (acetic acid) and NaCH3COO (sodium acetate). The buffer consists of a weak acid and its conjugate base.
Use the Henderson-Hasselbalch equation to calculate the pH of the buffer: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). Here, \([\text{A}^-]\) is the concentration of the conjugate base (NaCH3COO), and \([\text{HA}]\) is the concentration of the weak acid (CH3COOH).
Calculate the pKa from the given Ka value using the formula \( \text{pKa} = -\log(\text{Ka}) \). Substitute \( \text{Ka} = 1.8 \times 10^{-5} \) into the formula to find \( \text{pKa} \).
Substitute the concentrations of the acid and conjugate base into the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{1.3}{1.5} \right) \).
Simplify the expression to find the pH of the buffer solution. Remember that the logarithm of a fraction can be calculated using \( \log \left( \frac{a}{b} \right) = \log(a) - \log(b) \).
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