Chapter 16, Problem 21A

Interpretation Introduction

Interpretation: Effect of addition of acid on ${\text{OH}}^{-}$ ion in aqueous solution and effect of addition of base on ${\text{H}}^{+}$ ion in aqueous solution has to be explained.

Concept introduction: A very simple definition of acid is that any substance which furnishes ${\text{H}}^{\text{+}}$ ions when added to ${\text{H}}_2\text{O}$ can be put into a class of acid. On the counterpart if any substance accepts ${\text{H}}^{\text{+}}$ ions when added to ${\text{H}}_2\text{O}$ , can be classified as base.

Answer to Problem 21A

Effect of addition of acid on ${\text{OH}}^{-}$ ion in aqueous solution is that ${\text{OH}}^{-}$ ion concentration decreases. Similarly, effect of addition of base on ${\text{H}}_3{\text{O}}^{\text{+}}$ ion in aqueous solution is that ${\text{H}}_3{\text{O}}^{\text{+}}$ concentration decreases.

Explanation of Solution

Since water is amphoteric, one water molecule donates ${\text{H}}^{+}$

to another water molecule and vice versa. This leads to following reaction:

  ${\text{H}}_2\text{O}+{\text{H}}_2\text{O}\leftrightarrow {\text{H}}_3{\text{O}}^{+}+{\text{OH}}^{-}$

As soon as acid is added into water, due to common ion effect and water being a weak acid in comparison to acid, ionization of water gets suppressed. And hence ${\text{OH}}^{-}$ ion concentration decreases. This reaction proceeds:

  $\text{HA}+{\text{H}}_2\text{O}\leftrightarrow {\text{H}}_3{\text{O}}^{+}+{\text{A}}^{-}$

Similarly when base is added, formation of hydronium ion gets suppressed.

  $\text{BOH}+{\text{H}}_2\text{O}\leftrightarrow {\text{OH}}^{-}+{\text{B}}^{+}+{\text{H}}_2\text{O}$

Conclusion

Effect of addition of acid on ${\text{OH}}^{-}$ ion in aqueous solution is that ${\text{OH}}^{-}$ ion concentration decreases. Similarly, effect of addition of base on ${\text{H}}_3{\text{O}}^{\text{+}}$ ion in aqueous solution is that ${\text{H}}_3{\text{O}}^{\text{+}}$ concentration decreases.