You are asked to prepare a 100 mL buffer solution of pH=5.0. for this purpose, you are supplied with acetic acid `(pK_(a)=4.74)`, benzoic acid `(pK_(a)=4.20)`, formic acid `(pK_(a)=3.75)` and their sodium salts. Which one of the acids will you use to prepare the most effective buffer?

Explanation

The buffer action of an acidic buffer consisting of a weak acid and its salt becomes maximum when the pH of the solution becomes equal to the `pK_(a)` of the acid. The greater the difference between pH of the solution and `pK_(a)` of the acid, the lower is the buffer capacity of the solution. As given, pH of buffer solution=5.0. of the given acids, acetic acid has its value of `pK_(a)(4.74)` nearest to the value of pH of the solution. hence, acetic acid and its sodium salt will be used to prepare the buffer solution.