Question

One weak acid (like$\text{ C}{{\text{H}}_{\text{3}}}\text{COOH }$) and its strong base together with salt (like$\text{ C}{{\text{H}}_{\text{3}}}\text{COONa }$) is a buffer solution. In which pair the type of character is found:
A) $\text{ HCl }$ and $\text{ NaCl }$
B) $\text{ NaOH }$ and $\text{ NaN}{{\text{O}}_{\text{3}}}\text{ }$
C) $\text{ KOH }$ and $\text{ KCl }$
D) $\text{ N}{{\text{H}}_{\text{4}}}\text{OH }$ and $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$

Answer 1

Hint: A buffer solution is one that can resist change in its $\text{ pH }$ on the addition of an acid or a base. A buffer solution is a solution of a weak acid and its conjugate base or solution of a weak base and conjugate acid. The $\text{ pH }$change of the buffer solution to a very little extent when a small amount of strong acid (like $\text{ HCl }$ ) and strong bases ( like $\text{ NaOH }$ ) is added to it.

Complete step by step answer:
A buffer solution is one that can resist change in it’s $\text{ pH }$ on the addition of an acid or a base.
A very common buffer is prepared by mixing an equimolar aqueous solution of ammonium hydroxide $\text{ N}{{\text{H}}_{\text{4}}}\text{OH }$ and ammonium chloride $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$ . Ammonium hydroxide is slightly dissociated while ammonium chloride $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$ being a strong base salt is almost completely dissociated. The mixture thus contains ammonium hydroxide $\text{ N}{{\text{H}}_{\text{4}}}\text{OH }$ molecules as well as ammonium ion $\text{ NH}_{4}^{+}\text{ }$ and $\text{ C}{{\text{l}}^{-}}\text{ }$ ions. Let us consider a buffer solution of this mixture.
Suppose a strong base is added to the above mixture of ammonium hydroxide and ammonium chloride. The oxide ion $\text{ O}{{\text{H}}^{-}}\text{ }$ ions are added will be taken up immediately by ammonium ions to form very slightly dissociated$\text{ N}{{\text{H}}_{\text{4}}}\text{OH }$.
$\text{ NH}_{4}^{+}\text{ + O}{{\text{H}}^{-}}\text{ }\to \text{ N}{{\text{H}}_{\text{4}}}\text{OH }$
Thus the hydroxide ion added is neutralized by the ammonium ion present in the mixture. There is very little change in $\text{ pH }$ of the mixture.
If on the other hand a strong acid is added the $\text{ }{{\text{H}}^{\text{+ }}}$ ions added re neutralized by the ammonium hydroxide present in the mixture. The reaction is as follows,
$\text{ }{{\text{H}}^{\text{+ }}}+\text{ N}{{\text{H}}_{\text{4}}}\text{OH }\to \text{ NH}_{4}^{+}\text{ + }{{\text{H}}_{\text{2}}}\text{O }$
Thus, again there is very little change in the $\text{ pH }$ of the mixture.
Thus in the buffer solution of ammonium hydroxide and ammonium chloride reverse acidity is due to the presence of ammonium ion and reverse alkalinity is due to the presence of ammonium hydroxide.
Thus from the given pair $\text{ N}{{\text{H}}_{\text{4}}}\text{OH }$and $\text{ N}{{\text{H}}_{\text{4}}}\text{Cl }$ is a buffer solution.

Hence, (D) is the correct option.

Note: Note that only weak acid or bases and their conjugate base and acid forms buffer solutions. Consider a solution of sodium chloride water. It $\text{ pH }$is 7. The addition of even 1 m of 1 M $\text{ HCl }$ solution to one litre of sodium chloride solution lowers the $\text{ pH }$of solution from 7 to 3.Similarly, the addition of 1 ml of 1 M $\text{ NaOH }$ solution to one litre of sodium chloride solution raises the $\text{ pH }$of the solution from 7 to 11. So there is no little change in $\text{ pH }$instead of that there is a large difference in$\text{ pH }$. This sodium chloride is not a buffer solution.