Question at position 20 A 10 to 1 ratio of acetic acid to sodium acetate (pKa =4.54) is dissolved in 1 L of water. What is the pH of the resulting solution?A 10 to 1 ratio of acetic acid to sodium acetate (pKa =4.54) is dissolved in 1 L of water. What is the pH of the resulting solution?pH = 7pH = 5.54pH = 4.54pH = 3.54单项选择题
A
pH = 7
B
pH = 5.54
C
pH = 4.54
D
pH = 3.54

Question

Question at position 20 A 10 to 1 ratio of acetic acid to sodium acetate (pKa =4.54) is dissolved in 1 L of water. What is the pH of the resulting solution?A 10 to 1 ratio of acetic acid to sodium acetate (pKa =4.54) is dissolved in 1 L of water. What is the pH of the resulting solution?pH = 7pH = 5.54pH = 4.54pH = 3.54单项选择题

A

pH = 7

B

pH = 5.54

C

pH = 4.54

D

pH = 3.54

BlackTom AI · Accepted Answer

We start by identifying the buffer system and the given ratio. The solution contains acetic acid (HA) and its conjugate base acetate (A-), forming a standard weak acid–conjugate base buffer. The Henderson–Hasselbalch equation applies: pH = pKa + log([A-]/[HA]).

Option 1: pH = 7. This would correspond to a neutral pH, which is not characteristic of a buffer with a strong dominance of either HA or A-. Since the ratio is not 1:1, a neutral pH is unlikely.

Option 2: pH = 5.54. This would be the pH if the ratio [A-]/[HA] were 10:1 (log 10 = 1). However, the problem specifies a 10:1 ratio of acetic acid to sodium acetate, meaning [HA]/[A-] = 10, so [A-]/[HA] = 0.1, not 10.

Option 3: pH = 4.54. This would occur if [A-]/[HA] = 1, i.e., a 1:1 ratio, giving pH = pKa. The given ratio is not 1:1, so this is incorrect.

Option 4: pH = 3.54. Here, since the ratio [A-]/[HA] = 0.1 (because acetic acid is in 10:1 excess), log(0.1) = -1. Substituting into the equation gives pH = 4.54 + (-1) = 3.54, which matches the described mixture.

To summarize, the key is correctly interpreting the ratio direction in the Henderson–Hasselbalch equation. The 10:1 ratio of HA:A- translates to [A-]/[HA] = 0.1, yielding pH = pKa + log(0.1) = 4.54 - 1 = 3.54. The remaining options either assume a different ratio or ignore the log relationship, leading to incorrect pH values.

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