Question

A solution of ${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$ and ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ acts as a buffer. Justify.

Answer 1

We must know that ${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$ is a strong electrolyte hence, dissociates completely, ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ is a weak electrolyte.

Complete step-by-step answer:
We know that a buffer is a solution that resists the change in the pH even if it is diluted, or if relatively small amounts of strong acid or strong bases are added. That is, if we add a small quantity of an acid or base to a buffer solution it will change pH very little. A buffer solution can be made by mixing a weak acid/ weak base with one of its salts.
A weak base (${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$) and its salt with a strong acid (${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$) forms a basic buffer.
Let’s see how this mixture resists the change in$pH$.
${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$ is a strong electrolyte hence, dissociates completely into ions, while ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ is a weak electrolyte. The dissociation of ${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$ is further suppressed by common ion, ${\text{N}}{{\text{H}}_{\text{4}}}^{\text{ + }}\;$ provided by ${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$ in the solution.

$${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}} \to {\text{N}}{{\text{H}}_{\text{4}}}^{\text{ + }}{\text{ + C}}{{\text{l}}^{\text{ - }}} \\\ {\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}} \rightleftharpoons {\text{N}}{{\text{H}}_{\text{4}}}{^{\text{ + }}}{\text{ + O}}{{\text{H}}^{\text{ - }}} \\\$$

This solution is as basic a buffer as the pH of the solution is around 9.25. This mixture has the capability of resisting the change in pH upon addition of a small amount of acid or alkali.
Condition 1:- If a small amount of acid such as ${\text{HCl}}$ is added to this solution, then the ${{\text{H}}^{\text{ + }}}\;$ ions of the acid gets neutralized by ${\text{O}}{{\text{H}}^{\text{ - }}}$ ions already present in the solution. Moreover, ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ molecules get ionized to compensate loss of ${\text{O}}{{\text{H}}^{\text{ - }}}$ ions. Therefore, the pH of the solution practically remains constant.
Therefore, we can conclude that the solution of ${\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}$and ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ can acts as a buffer.
Condition 2:- If a small amount of base such as ${\text{NaOH}}\;$ is added to this solution, ${\text{O}}{{\text{H}}^{\text{ - }}}$ ions of ${\text{NaOH}}\;$ combine with ${\text{N}}{{\text{H}}_{\text{4}}}^{\text{ + }}$ ions present in large number forming weakly ionized ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$. Thus, the pH of the solution remains practically unchanged.

Note:
We can use buffer solutions for different applications. In alcohol production, it helps to keep the PH at a specific level when we convert sugar to alcohol. In dyeing fabrics, it helps to maintain the specific pH because the wrong pH will affect the colour of the dye. Also we can use it in household products, cosmetics, printings and calibrating equipment.
An example of acidic buffer is weak acid with a salt of the same acid with a strong base. Example of an acidic buffer:-${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH + C}}{{\text{H}}_{\text{3}}}{\text{COONa}}$.