Chapter U6, Problem 17STP

Interpretation Introduction

Interpretation:

The correct statement about nitrous acid, nitric acid and benzoic acid on the basis of given data needs to be determined.

Concept introduction:

An acid is the substance that gives H+ or ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ ions in its aqueous solution. A weak acid ionized partially and reaches to equilibrium.

${\text{HA}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{+A}}^{\text{-}}{}_{\text{(aq)}}$

The acid dissociation constant is the ratio of concentration of product and reactant for the equilibrium reaction of weak acid in solution. For the given weak acid HA, it can be written as:

$\begin{array}{l}{\text{HA}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{+A}}^{\text{-}}{}_{\text{(aq)}}\\ {\text{K}}_{\text{a}}\text{=}\frac{{\text{[H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{] [A}}^{\text{-}}\text{]}}{\text{[HA]}}\end{array}$

Answer to Problem 17STP

Option (a) is correct.

Explanation of Solution

Reason for correct option:

A strong acid dissociates completely in water and gives hydronium ions in the solution. Nitric acid is a strong acid as it has high value of acid dissociation constant as given. Thus, the ionization of nitric acid can be written as:

${\text{HNO}}_{\text{3}}{}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{+NO}}_{\text{3}}{{}^{\text{-}}}_{\text{(aq)}}$

Nitrous acid and benzoic acid are weak acid as they have less value of acid dissociation constant therefore they will ionized partially in solution;

$\begin{array}{l}{\text{HNO}}_2{}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{+NO}}_2{{}^{\text{-}}}_{\text{(aq)}}\\ {\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{COOH}}_{}{}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{+ C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{COO}}^{\text{-}}{}_{\text{(aq)}}\end{array}$

Conclusion

Reasons for incorrect options: Nitric acid is a strong acid therefore it will ionized completely, whereas nitrous acid and benzoic acid are weak acid therefore they will dissociate partially.

Thus option (B), (C) and (D) are incorrect.