Question

A buffer solution is prepared by mixing 0.1 mol of acetic acid (\( \mathrm{p}K_a = 4.74 \)) and 0.2 mol of sodium acetate in 1 L solution. What is the pH of the buffer?

• A. 4.44
• B. 5.04
• C. 4.74
• D. 5.74

Hint:

\textbf{Tip:} For weak acid–salt buffers, use the Henderson-Hasselbalch equation and remember \( \log 2 \approx 0.30 \).

Answer 1

The Correct Option is B

The pH of the buffer solution is determined using the Henderson-Hasselbalch equation:

\[ \text{pH} = \text{p}K_a + \log{\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)} \]

Given:
\( \text{p}K_a = 4.74 \) (acetic acid),
\([\text{A}^-]\) = 0.2 mol/L (acetate ion),
\([\text{HA}]\) = 0.1 mol/L (acetic acid).

Substituting the values:

\[ \text{pH} = 4.74 + \log{\left(\frac{0.2}{0.1}\right)} \]

Simplifying the expression:

\[ \text{pH} = 4.74 + \log{(2)} \]

Using \( \log{(2)} \approx 0.301 \):

\[ \text{pH} = 4.74 + 0.301 = 5.04 \]

The pH of the buffer solution is 5.04.