Henderson equation for acid buffer solution
pH=pKa+log [salt]/[acid]
Let the volume of benzoic acid solution to be add benzoic acid solution is xL to make 300mLor 0.3L .So the volume of benzoic acid solution will be (0.3-x)L.
Hence the concentration of salt in the mixture = [salt]=0.2(0.3-x)/0.3
And the concentration of acid in the mixture [acid]= 0.1x/0.3
So [salt]/[acid]=2(0.3-x)/x
So we have
3.9=4.2+log([salt]/[acid])
=>0.3=log([acid]/[salt])=log(x/(2(0.3-x))
=>x/(2(0.3-x))=2
=>x=1.2-4x
=>5x=1.2
=>x=0.24L=240mL
Option A is valid
