Chapter 19, Problem 87A

Interpretation Introduction

Interpretation : The three equations for the stepwise ionization of phosphoric acid are to be written.

Concept Introduction : Acids are substances that contain hydrogen and ionize to produce hydrogen ions $\left({\text{H}}^{\text{+}}\right)$ in an aqueous solution.

Answer to Problem 87A

The three equations for the stepwise ionization of phosphoric acid are given:

  $\begin{array}{l}{\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)\hfill \\ {\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{HPO}}_4^{2-}\left(aq\right)\hfill \\ {\text{HPO}}_4^{2-}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{PO}}_4^{3-}\left(aq\right)\hfill \end{array}$

Explanation of Solution

The acid dissociation constant is the ratio of an acid's dissociated concentration to its undissociated form concentration. It is denoted as ${\text{K}}_{\text{a}}$ .

Phosphoric acid has three hydrogens which go as hydrogen ions in an aqueous solution. The dissociation of phosphoric acid in an aqueous solution takes place in three steps.

The three steps are as follows:

  $\begin{array}{l}{\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)\hfill \\ {\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{HPO}}_4^{2-}\left(aq\right)\hfill \\ {\text{HPO}}_4^{2-}\left(aq\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(aq\right)+{\text{PO}}_4^{3-}\left(aq\right)\hfill \end{array}$