Question

$pH$ of a mixture of $1 M$ benzoic acid $\left( pK _{ a }=4.20\right)$ and $1 M C _{6} H _{5} COONa$ is $4.5$, what is the volume of benzoic acid required to prepare a $300 \,ml$ buffer $[\log 2=0.3]$ ?

• A. 200 ml
• B. 150 ml
• C. 100 ml
• D. 50 ml

Answer 1

Answer: (C)

$pH = pKa +\log \frac{ V }{300- V } $
$4.5=4.2+\log \frac{ V }{300- V }$
$0.3=\log \frac{ V }{300- V }$
$\log 2=\log \frac{ V }{300- V }$
$\therefore 2=\frac{ V }{300- V }$
$\therefore V =200 $
$\therefore $ Volume of the acid $=300-200=100$