Chapter 13, Problem 48QP

Interpretation Introduction

Interpretation:

The ions or molecules present in the aqueous solution of the oxalic acid are to be determined. The identification of the ion that is present in the lowest concentration id to be done.

Concept Introduction:

The chemical substance which releases hydrogen ion in the solution is known as acidic substance. In such compounds, the hydrogen atom is attached to an electronegative atom. Thus, it readily loses hydrogen ions in the solution.

Explanation of Solution

Citric acid is a weak organic acid and furnishes hydronium ions according to its $K_{\text{a}}$ value.

The equations of the ionization of citric acid are:

$\begin{array}{l}{\text{H}}_3{\text{C}}_6{\text{H}}_5{\text{O}}_7(aq)+{\text{ H}}_2\text{O}(l)\rightleftharpoons {\text{ H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}(aq)+{\text{ H}}_3{\text{O}}^{+}(aq)K_{{\text{a}}_1}=7.4\times {10}^{-4}\\ {\text{H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}(aq)+{\text{ H}}_2\text{O}(l)\rightleftharpoons {\text{ HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}(aq)+{\text{ H}}_3{\text{O}}^{+}(aq)K_{{\text{a}}_2}=1.7\times {10}^{-5}\\ {\text{HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}(aq)+{\text{ H}}_2\text{O}(l)\rightleftharpoons {\text{ C}}_6{\text{H}}_5{\text{O}}_7{}^{3-}(aq)+{\text{ H}}_3{\text{O}}^{+}(aq)K_{{\text{a}}_3}=4.0\times {10}^{-7}\end{array}$

It is apparent from the equations that the molecules present in the solution are ${\text{H}}_3{\text{C}}_6{\text{H}}_5{\text{O}}_7$ and ${\text{ H}}_2\text{O}$ and the ions present are ${\text{H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}$ , ${\text{HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}$ , ${\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{3-}$ and ${\text{H}}_3{\text{O}}^{+}$ . It is evident from the $K_{\text{a}}$ values that the concentration of ${\text{H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}$ is low and that of ${\text{HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}$ is lower and it is the lowest for the ${\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{3-}$ ions as is seen from the $K_{\text{a}}$ values.

Conclusion

The molecules present in the solution are ${\text{H}}_3{\text{C}}_6{\text{H}}_5{\text{O}}_7$ and ${\text{ H}}_2\text{O}$ and the ions present are ${\text{H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}$ , ${\text{HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}$ , ${\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{3-}$ and ${\text{H}}_3{\text{O}}^{+}$ . The concentration of ${\text{H}}_2{\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{-}$ is low and that of ${\text{HC}}_6{\text{H}}_5{\text{O}}_7{}^{2-}$ is lower and it is the lowest for the ${\text{C}}_6{\text{H}}_5{\text{O}}_7{}^{3-}$ ions.