Chapter 16, Problem 47A

Interpretation Introduction

Interpretation:

The component of buffer solution which is capable of combining with strong acid $HCl$ is to be written by giving example.

Concept introduction:

A solution is buffer if it does not change the $pH$ value if small amount of acid or base is added to it.

Buffer solution is of two types:

• Acidic buffer
• Basic buffer

Answer to Problem 47A

In $C{H}_{3}COOH-C{H}_{3}COONa$ and

In $N{H}_{4}OH-N{H}_{4}Cl$ buffer, $HCl$ reacts with ammonium hydroxide to neutralise the effect of addition.

Explanation of Solution

  $C{H}_{3}COOH-C{H}_{3}COONa$ is acidic buffer.

It works as:

  $\begin{array}{l}C{H}_{3}COOH\rightleftharpoons C{H}_{3}CO{O}^{-}+H^{+}\\ C{H}_{3}COONa\to C{H}_{3}CO{O}^{-}+N{a}^{+}\end{array}$

When small amount of acid is added:

  $HCl\to H^{+}+C{l}^{-}$

Common ion effect operates and equilibrium of acetic acid is shifted backward and thus effect of acid is neutralised.

  $N{H}_{4}OH-N{H}_{4}Cl$ is a basic buffer.

It works as:

  $\begin{array}{l}N{H}_{4}OH\rightleftharpoons N{H}_4{}^{+}+O{H}^{-}\\ N{H}_{4}Cl\to N{H}_4{}^{+}+C{l}^{-}\end{array}$

When small amount of acid is added, it reacts with ammonium hydroxide and get neutralised.

  $HCl\to H^{+}+C{l}^{-}$

These hydrogen ions are neutralised by hydroxyl ions furnished by ammonium hydroxide.

  $H^{+}+O{H}^{-}\to H_{2}O$

Conclusion

Thus, $pH$ of a buffer solution is fixed. On addition of small electrolyte in it, there is only slight change in $pH$ .