Chapter 16, Problem 46A

Interpretation Introduction

Interpretation:

The components of buffer solution are 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 46A

The component necessary for buffer solution is

• Weak acid and a salt of weak acid with strong base.
• Weak base and a salt of weak base with strong acid.

Example: $C{H}_{3}COOH-C{H}_{3}COONa$ and $N{H}_{4}OH-N{H}_{4}Cl$

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 base is added, it reacts with acetic acid and get neutralised.

When small amount of acid is added, 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.

When small amount of base is added, common ion effect operates and equilibrium of ammonium hydroxide is shifted backward and thus effect of base is neutralised.

Conclusion

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