In a weak acid calculation, if [H₃O⁺] = [A⁻], what is true about the concentration of the acid?

In this context, if the concentration of hydronium ions ([H₃O^+]) is equal to the concentration of the conjugate base ([A^-]), it reflects an equilibrium situation characteristic of weak acids. Weak acids do not fully dissociate in solution; instead, they exist in a state where only a fraction of the acid molecules release protons, establishing a balance between the undissociated acid, ([HA]), and its dissociation products, ([H₃O^+]) and ([A^-]).

Insisting that the acid concentration remains unchanged indicates that while some acid has dissociated to produce ([H₃O^+]) and ([A^-]), the overall concentration of the acid does not decrease significantly enough to be noted; thus, its presence in solution is predominant compared to the amounts of dissociated ions. Therefore, the concentration of the weak acid can be considered approximately constant when calculating equilibrium concentrations, which allows for manageable calculations and justification when considering weak acid equilibria.

This reasoning elucidates why the concentration of the weak acid remains substantially unchanged during the dissociation process.