It is desired to prepare 100 mL of a buffer of pH 5.00. Acetic ( pK_a = 4.74), benzoic ( pK_a = 4.18) and formic acids (pK_a = 3.68) and their salts are available for use. Which acid should be used for maximum effectiveness against increase or decrease in PH?

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  ${\mathrm{pK}}_{\mathrm{a}}$ of the acid. The greater the difference between pH of the solution and ${\mathrm{pK}}_{\mathrm{a}}$ 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 ${\mathrm{pK}}_{\mathrm{a}}(4.74)$ nearest to the value of pH of the solution. hence, acetic acid and its sodium salt will be used to prepare the buffer solution.