Question

pH of a mixture of 1 M benzoic acid $(p{K}_a=4.20)$ and 1M $C_6{H}_{5}COONa$ 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)

100 mL

As we know, $pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}$

Here, $4.5=4.2+log\left(\frac{1\times V}{1\times v}\right)$

V = volume of salt

v =volume of acid

$0.3=log\left(2\right)=log\left(\frac{V}{v}\right)$

Total volume of buffer $=300ml=V+v$ and $\frac{V}{v}=2$

So by solving these two equations,

$v=100ml$

$V=200ml$