Question

Fixed volume of $0.1 \, M$ benzoic acid solution is added into $0.2 \, M$ sodium benzoate solution and formed a $300 \, ml$ , resultant acidic buffer solution. If $pH$ of this buffer solution is $4.5$ then find added volume of benzoic acid -

$(Given:pK_{a} \, \text{benzoic} \, \text{acid} = 4.2)$

• A. $100 \, ml$
• B. $150 \, ml$
• C. $200 \, ml$
• D. None of these

Answer 1

Answer: (B)

$pH=pK_{a}+log \frac{s a l t}{a c i d}$

$4.5=4.2+log \frac{s a l t}{a c i d}$

$0.3=log \frac{s a l t}{a c i d}$

$\frac{s a l t}{a c i d}=2$

$\frac{0.3 \times \left(300 - v\right)}{0.1 \times v}=2$

$v=150 \, ml$