Which of the following acids would be needed to combine with its potassium salt to produce a solution buffered at pH 5.00?
Chlorous acid: pK_a = 1.95
Nitrous acid: pK_a = 3.34
Formic acid: pK_a = 3.74
Acetic acid: pK_a = 4.74
Hypochlorous acid: pK_a = 7.54

• A. Chlorous acid
• B. Nitrous acid
• C. Formic acid
• D. Acetic acid
• E. Hypochlorous acid
• F. None of the acids would buffer at pH 5.00.

Answer: (D)

Verified step by step guidance

1. Understand the concept of a buffer solution: A buffer solution is made from a weak acid and its conjugate base (often a salt) and resists changes in pH when small amounts of acid or base are added.
2. Identify the relationship between pH, pKa, and the buffer system: The pH of a buffer is related to the pKa of the acid and the ratio of the concentrations of the conjugate base to the acid, as described by the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \).
3. Determine the desired pH for the buffer: In this problem, the desired pH is 5.00.
4. Select the appropriate acid: To create a buffer at a specific pH, choose an acid whose pKa is close to the desired pH. This ensures that the ratio \( \frac{[\text{A}^-]}{[\text{HA}]} \) is close to 1, making the buffer most effective.
5. Compare the given pKa values to the desired pH: The pKa of acetic acid (4.74) is closest to the desired pH of 5.00, making it the best choice for creating a buffer at this pH.