Question

A buffer solution is prepared by mixing 10 mL of 1.0 M acetic acid and 20 mL of 0.5 M sodium acetate acid and then diluted to 100 mL with distilled water. If the $pK_a$ of $CH_3COOH$ is 4.76, what is the pH of the buffer solution prepared ?

• A. 5.21
• B. 4.76
• C. 4.34
• D. 5.22

Answer 1

Answer: (B)

4.76

Answer 2

The expression for the $pH$ of the acidic buffer solution is as given below. $pH = pK _{ a }+\log \frac{[\text { Salt }]}{[\text { Acid }]}$ $pK _{ a }=4.76$ [Salt] $=\frac{20 \times 0.5}{100}=0.1 M$ [Acid] $=\frac{10 \times 1}{100}=0.1 M$ Substitute values in the above equation. $pH =4.76+\log \frac{0.1}{0.1}=4.76$