Question

An aqueous buffer is prepared by adding $100\, mL$ of $0.1\, mol\, L ^{-1}$ acetic acid to $50\, mL$ of $0.2 \,mol\, L ^{-1}$ of sodium acetate. If $p K_{ a }$ of acetic acid is $4.76$, the $pH$ of the buffer is

• A. 4.26
• B. 5.76
• C. 3.76
• D. 4.76

Answer 1

Answer: (D)

$M_{\text {eq }}$ of $CH _{3} COOH =100 \times 0.1=10$
$M_{\text {eq }}$ of $CH _{3} COONa =50 \times 0.2=10$
According to Henderson equation
$pH = p K_{a}+\log \frac{[\text{ salt} ]}{[\text { acid }]}$
$pH = p K_{a}+\frac{\log \left[ CH _{3} COO ^{-}\right]}{\log \left[ CH _{3} COOH \right]}$
$pH =4.76+\log \frac{10}{10} $
$pH =4.76+\log 1$
$\Rightarrow pH =4.76$