Which of the following is not a buffer solution?

The correct option is A

$(\mathrm{HCl}/\mathrm{NaCl})$

Explanation for correct option:

A. $(\mathrm{HCl}/\mathrm{NaCl})$

Buffer solution:

• A buffer solution is a water solvent-based solution that is made up of a weak acid and its conjugate base or a weak base and its conjugate acid.
• In buffer solution, the pH remains unaltered by the addition of acid/base.
• An example of a buffer solution is a solution of acetic acid$\left({\mathrm{CH}}_3\mathrm{COOH}\right)$ and sodium acetate$\left({\mathrm{CH}}_3\mathrm{COONa}\right)$.

$(\mathrm{HCl}/\mathrm{NaCl})$this is not a buffer solution as hydrochloric acid is a strong acid.

Explanation for incorrect options:

B. $({\mathrm{NH}}_4\mathrm{OH}/{\mathrm{NH}}_4\mathrm{Cl})$it is a buffer solution as ammonium hydroxide is a weak base and its conjugate acid is ammonium chloride.

C. $(\mathrm{HCOOH}/\mathrm{HCOONa})$ it is a buffer solution as formic acid is a weak acid and its conjugate base is sodium formate.

D. $({\mathrm{CH}}_3\mathrm{COOH}/{\mathrm{CH}}_3\mathrm{COONa})$ it is a buffer solution as acetic acid is a weak acid and its conjugate base is sodium acetate.

Hence, the correct option is A i.e. $(\mathrm{HCl}/\mathrm{NaCl})$is not a buffer solution.