Question

Benzoic acid is a weak acid with a pKa of 4.19. Which among the following options best represent the nature of benzoic acid.
A. A strong acid
B. A strong base
C. Neutral
D. All of these.

Answer 1

Hint: pH is a scale used to specify the acidity or basicity of an aqueous solution. The pH of benzoic acid is 4.19 and the higher pKa value means weaker acid or stronger base. And we also know that pKa+pKb=14. So pKb of benzoic acid is 9.81.

Complete step-by-step answer:
In this question we have asked, at pH of 4.19 benzoic acid acts as a strong acid, strong base or neutral. As we know, benzoic acid is an organic acid and is a weak acid as given in the question. Acid means it easily gives H+ in the aqueous solution. The easier and faster the rate of giving protons, the more strong the acid is. Due to the resonance structure of benzoic acid, power of leaving proton is restricted and that's why its ability to leave proton is decreased.
 As we know, A larger pKa for the conjugate acid means a stronger base and for measuring the base strength, we use pKb, in which case a smaller value is more basic. As we can see benzoic acid is a weak acid. So, it will dissociate partially in the solution and we know that the conjugate base of a weak acid acts as a strong base. So, we can say benzoic acid is a weak acid and strong base. So, the correct option will be B. A strong base.

The correct option is B. A strong base.

Note:In case of benzoic acid, the carboxylic acid group is directly attached to the sp2 hybridized carbon atom. As we know, the +R effect of phenyl groups is greater than –I effect ( +R > –I ). So, the O–H bond of benzoic acid becomes less polar. Due to which, the tendency of benzoic acid to donate protons decreases.