Question

Calculate $pH$ of a buffer prepared by adding $10 \,mL$ of $0.10 \,M$ acetic acid to $20\, mL$ of $0.1\, M$ sodium acetate [$pK_a (CH_3COOH) = 4.74]$

• A. 3
• B. 4.44
• C. 4.74
• D. 5.04

Answer 1

Answer: (D)

5.04

Answer 2

The correct answer is D:5.04
The number of millimoles of $CH_3COOH$ is calculated as $0.1 \times 10 = 1.0$, while the millimoles of $CH_3COONa$ is calculated as $0.1 \times 20 = 2.0.$
Using the Henderson-Hasselbalch equation:
$pH = pK_a +log\frac{[conjugate\space base]}{[acid]}$
$= 4.74 + log(\frac{2}{1})$
$=4.47+0.30=5.04$