Question

Which of the following equimolar mixture can act as a salt buffer ?

• A. $C{H}_{3}COOH+C{H}_{3}COONa$
• B. $N{H}_{4}OH+N{H}_{4}Cl$
• C.
  $N{H}_{4}OH+C{H}_{3}COOH$
• D. $HCl+NaOH$

Answer 1

The correct option is C

$N{H}_{4}OH+C{H}_{3}COOH$
$C{H}_{3}COOH$ is a weak acid and $C{H}_{3}CO{O}^{-}$ is the conjugate base of the weak acid. So the acidic buffer solution can be formed by the combination of weak acid and weak base.

$N{H}_{4}OH$ is a weak base. $N{H}_4^{+}$ is the conjugate acid of weak base. So, the resultant solution can acts as a basic buffer solution.

A solution of the salt of a weak acid (WA) and a weak base (WB) in water is called a salt buffer.
Example :
$C{H}_{3}CO{O}^{-}\left(aq\right)+N{H}_4^{+}\left(aq\right)+H_{2}O\left(l\right)\rightleftharpoons C{H}_{3}COOH\left(aq\right)+N{H}_{4}OH\left(aq\right)$
If an acid (extra $H^{+}$ is added to the buffer, $C{H}_{3}CO{O}^{-}$ neutralises it
$H^{+}\left(aq\right)+C{H}_{3}CO{O}^{-}\left(aq\right)\rightleftharpoons C{H}_{3}COOH\left(aq\right)$
If a base (extra $O{H}^{-}$ is added to the buffer, $N{H}_4^{+}$ neutralises it
$O{H}^{-}\left(aq\right)+N{H}_4^{+}\left(aq\right)\rightleftharpoons N{H}_{4}OH\left(aq\right)$
The salt solution itself acts as a buffer, known as a salt buffer.


A mixture of strong acid ($HCl$) and strong base ($NaOH$) is not a buffer since the species are both strong they would completely neutralize each other leaving a salt and water without any molecule remaining to provide buffering action. Therefore this is not a buffer.