The pKa for CH3COOH is 4.75. Hence mixture of sodium acetate and acetic acid can be used for preparing buffer solution whose pH is in the range:

Question

Accepted Answer

To understand why Option 3 is the correct answer, we need to delve into the concept of buffer solutions and the role of pKa in determining the effective pH range of a buffer.

1. **Explanation for Option 3 (3.75 to 5.75):**

A buffer solution is most effective when the pH is close to the pKa of the acid involved. The pKa of acetic acid (CH3COOH) is 4.75. A buffer solution typically works best within a range of ±1 pH unit from the pKa value. This is because, within this range, the concentrations of the acid and its conjugate base (in this case, acetate ion, CH3COO-) are sufficient to resist changes in pH upon the addition of small amounts of acid or base.

Therefore, for acetic acid with a pKa of 4.75, the effective buffering range is approximately from 3.75 to 5.75. This is why Option 3 is correct, as it falls within this effective buffering range.

2. **Explanation for why other options are incorrect:**

- **Option 1 (0 to 4.75):** This range includes pH values that are too low for the effective buffering capacity of the acetic acid/acetate system. While the upper limit of 4.75 is the pKa, the lower limit of 0 is far below the effective buffering range, where the solution would not effectively resist changes in pH.

- **Option 2 (4.75 to 14):** Although the lower limit of this range starts at the pKa, the upper limit extends far beyond the effective buffering range. A pH of 14 is highly basic, and the acetic acid/acetate buffer would not be effective at such high pH values. The buffering capacity diminishes significantly beyond 5.75.

- **Option 4 (1 to 14):** This range is too broad and includes pH values where the buffer would not be effective. The lower end (pH 1) is too acidic, and the upper end (pH 14) is too basic for the acetic acid/acetate buffer system to maintain a stable pH.

In conclusion, Option 3 is the correct choice because it accurately represents the effective buffering range of a sodium acetate and acetic acid mixture, which is centered around the pKa of acetic acid. This range ensures that the buffer can effectively resist changes in pH, making it the most suitable option.