Which of the following represents the acid dissociation constant (Ka)?

The acid dissociation constant, represented as ( K_a ), is a quantitative measure of the strength of an acid in solution. It specifically indicates how completely an acid can donate a proton (H⁺) to a base in a chemical reaction.

In general, the dissociation of a weak acid can be represented as:

[ HA \rightleftharpoons H^+ + A^- ]

Here, ( HA ) is the undissociated acid, while ( H^+ ) is the hydrogen ion produced, and ( A^- ) represents the conjugate base. The expression for ( K_a ) is derived from the equilibrium concentrations of the products and the reactants at equilibrium:

[ K_a = \frac{[H^+][A^-]}{[HA]} ]

This formula shows that ( K_a ) is calculated by taking the concentration of the products (the hydrogen ion and the conjugate base) and dividing by the concentration of the reactant (the undissociated acid). This highlights that the correct representation for the acid dissociation constant ( K_a ) involves the concentrations of products over the undissociated acid, confirming that the second choice accurately represents this relationship