Chapter 16, Problem 48A

Interpretation Introduction

Interpretation:

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

Concept introduction:

A solution is buffer which resists the change in $pH$ value when a small amount of acid or base is added to it.

Buffer solution is of two types:

• Acidic buffer
• Basic buffer

Answer to Problem 48A

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:

  $NaOH\to N{a}^{+}+O{H}^{-}$

These hydroxyl ions are neutralised by hydrogen ions furnished by acetic acid as:

  $H^{+}+O{H}^{-}\to H_{2}O$

  $N{H}_{4}OH-N{H}_{4}Cl$ 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 base is added, hydroxyl ion becomes common. So, 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 $pH$ .