1 M benzoic acid `(pK_a=4.20)` and 1 M `C_6H_5COONa` solutions are given separately What is the volume of benzoic acid required to prepare a 300 ml buffer solution of pH =4.5 ? [log 2 =0.3]

Options

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

Answer

C

Explanation

`pH=pK_(a)+log``(["Salt"])/([Acid])`
`:. 4.5=4.2+log``(["Salt"])/([Acid])=log``(["Salt"])/([Acid])=0.3`
(since `log 2=0.3`) `:. (["Salt"])/([Acid])=2`
Let `Vml 1 M C_(6)H_(5)COOH` solution and `(300-V) 1 M C_(6)H_(5)COONa` solution be mixed together
`[Acid]=(Vxx1)/(1000)xx(1000)/(300)` , `[Salt]=(300-V)/(1000)xx(1000)/(300)`
`[Acid]=(V)/(300)`, `["Salt"]=(300-V)/(300)`
`:. (300-V//300)/(V//300)=2=300-V=2V :. V=100 ml`