Chapter U6, Problem 18STP

Interpretation Introduction

Interpretation:

The correct expression for the equilibrium constant for the dissociation of benzoic acid needs to be determined.

Concept introduction:

For an equilibrium reaction as follows:

$A+B\rightleftharpoons AB$

The expression for equilibrium constant is as follows:

$K=\frac{\left[AB\right]}{\left[A\right]\left[B\right]}$

Here, $\left[A\right],\left[B\right]\text{ and }\left[AB\right]$ is equilibrium concentration of A, B and AB.

Answer to Problem 18STP

Option (c)

Explanation of Solution

The benzoic acid is a weak acid thus, it does not completely dissociate into ions. The dissociation reaction is represented as follows:

${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{COOH}\rightleftharpoons {\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{COO}}^{-}+{\text{H}}^{+}$

Since, the expression for equilibrium constant is ratio of concentration of product species to concentration of reactant species raised to their stoichiometric coefficient thus, the expression for equilibrium constant will be:

$K_a=\frac{\left[{\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{COO}}^{-}\right]\left[{\text{H}}^{+}\right]}{\left[{\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{COOH}\right]}$

Therefore, option (c) is correct.