Question

Assertion : Benzoic acid is stronger acid than acetic acid.
Reason : ${{\text{K}}_{\text{a}}}$ for benzoic acid is $6.5 \times {10^{ - 5}}$ and for acetic acid is $1.74 \times {10^{ - 5}}$
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
(D) Both assertion and reason are incorrect.

Answer 1

Hint : Different concepts have been given by different investigators in order to characterise acids. In general we can say that an acid is a hydrogen containing substance and is capable of donating proton / hydrogen ions to other substances. An acid can be identified by its sour taste and it will turn blue litmus to red colour.
Consider an equilibrium reaction of acid
${\text{HA}}\left( {aq} \right)\, + \,{{\text{H}}_2}{\text{O}}\left( l \right)\, \rightleftharpoons \,{{\text{H}}_3}{{\text{O}}^ + }\left( {aq} \right)\, + \,{{\text{A}}^ - }\left( {aq} \right)$
From above equilibrium reaction we can write the acid ionisation constant as
${{\text{K}}_a}\, = \,\dfrac{{\left[ {{{\text{H}}_3}{{\text{O}}^ + }} \right]\left[ {{{\text{A}}^ - }} \right]}}{{\left[ {{\text{HA}}} \right]}}$
$\left[ {{{\text{H}}_3}{{\text{O}}^ + }} \right]\, - \,{\text{concentration of }}\,{{\text{H}}_3}{{\text{O}}^ + } \\\ \left[ {{{\text{A}}^ - }} \right]\, - \,{\text{concentration of }}{{\text{A}}^ - } \\\ \left[ {{\text{HA}}} \right]\, - \,{\text{concentration of HA}} \\\$
Here although water is the reactant in this reaction , it is the solvent as well so that we do not include the concentration of ${{\text{H}}_2}{\text{O}}$ .

Complete Step By Step Answer:
Acid strength is a measure of the ability of the acid to lose its ${{\text{H}}^ + }$ ion. As we know acid strength is different for different acids. An acid that has greater degree of dissociation behaves has a stronger acid. The relative strength of acids can be determined by measuring their equilibrium constant in aqueous solution. In solutions of the same concentration, stronger acids will ionize to a greater extent and concentrations of hydronium ion ( ${{\text{H}}_3}{{\text{O}}^ + }$ ) will be higher.
For a reaction of acid ${\text{HA}}$ ,
${\text{HA}}\left( {aq} \right)\, + \,{{\text{H}}_2}{\text{O}}\left( l \right)\, \rightleftharpoons \,{{\text{H}}_3}{{\text{O}}^ + }\left( {aq} \right)\, + \,{{\text{A}}^ - }\left( {aq} \right)$
We can write the ionization constant ${{\text{K}}_a}\, = \,\dfrac{{\left[ {{{\text{H}}_3}{{\text{O}}^ + }} \right]\left[ {{{\text{A}}^ - }} \right]}}{{\left[ {{\text{HA}}} \right]}}$
The larger the value of ${{\text{K}}_a}$ for an acid then larger will be the concentration of ${{\text{H}}_3}{{\text{O}}^ + }$ . So stronger will be the acid. So we can say that larger the ionization constant stronger will be the acid.
Here ${{\text{K}}_{\text{a}}}$ value of benzoic acid is given by $6.5 \times {10^{ - 5}}$ and for acetic acid it is $1.74 \times {10^{ - 5}}$ .
So here benzoic acid has a higher ${{\text{K}}_{\text{a}}}$ value than acetic acid which indicates that benzoic acid is stronger acid than acetic acid.
Hence the correct answer is option is A.

Note :
A strong acid one which is completely ionized in the solution whereas a weak acid is partially ionized. The degree of dissociation of acid depends on factors such as strength and polarity of ${\text{H - A}}$ bond. Weaker the strength and greater the polarity of the ${\text{H - A}}$ bond stronger will be the acid.