Question

question 3 (3 points)
the following is list of weak acids and their pkₐ values
weak acid pkₐ
benzoic acid 4.20
hydrogen citrate 6.40
hydrocyanic acid 9.31
hydrogen phosphate 12.38
which would be the best buffer to use at ph 7? 1
for benzoic acid, a 50:50 mixture of benzoic acid and its conjugate base, the benzoate ion, would have ph = 2
the buffering capacity of a benzoic acid:benzoate buffer would be 3 at ph = 7
a. 4.20 b. 6.40 c. 9.31 d. 12.38 e. benzoic acid f. hydrogen citrate
g. hydrocyanic acid h. hydrogen phosphate i. good j. bad

Explanation:

Step1: Determine the best buffer at pH 7

A buffer works best when the pH is close to its \( pK_a \). We calculate the difference between pH (7) and each \( pK_a \):

• Benzoic acid: \( |7 - 4.20| = 2.80 \)
• Hydrogen citrate: \( |7 - 6.40| = 0.60 \)
• Hydrocyanic acid: \( |7 - 9.31| = 2.31 \)
• Hydrogen phosphate: \( |7 - 12.38| = 5.38 \)

The smallest difference is for hydrogen citrate (0.60), so the best buffer at pH 7 is hydrogen citrate (F).

Step2: pH of 50:50 benzoic acid - benzoate buffer

For a buffer with equal concentrations of acid and conjugate base (50:50), \( pH = pK_a \) (Henderson - Hasselbalch equation: \( pH = pK_a + \log\frac{[A^-]}{[HA]} \), when \( [A^-] = [HA] \), \( \log(1)=0 \), so \( pH = pK_a \)). The \( pK_a \) of benzoic acid is 4.20 (A), so \( pH = 4.20 \).

Step3: Buffering capacity at pH 7 for benzoic acid - benzoate buffer

Buffering capacity is best near \( pK_a \). The \( pK_a \) of benzoic acid is 4.20, and pH = 7 is far from 4.20, so the buffering capacity is bad (J).

Answer:

1. F. hydrogen citrate
2. A. 4.20
3. J. bad