Question

$pH$ of a buffer prepared by adding $10 \,mL$ of $0.10 \,M$ acetic acid to $20\, mL$ of $0.1 \,M$ sodium acetate is equal to $\left( p K_{a}\right.$ of $\left.CH _{3} COOH =4.74\right)$

• A. $4.74$
• B. $5.6$
• C. $5.04$
• D. $6.02$

Answer 1

Answer: (C)

We can easily calculate the $pH$ by converting the concentration into millimoles.
Millimoles of $CH _{3} COOH =0.1 \times 10=1.0$ millimole
Millimoles of $CH _{3} COONa =0.1 \times 2.0=2.0$ millimole
$\therefore$ Using Henderson-Hassebalch equation,
$pH = p K_{a}+\log \frac{\text { [Conjugate base }]}{[\text { acid }]}$
$=4.74+\log \frac{2}{1}$
$=4.74+0.30$
$=5.04$