Sketch two graphs: (a) percent dissociation for weak acid HA...

Sketch two graphs: (a) percent dissociation for weak acid HA versus the initial concentration of $\mathrm{HA}_{\text {(fHA }}^{\mathrm{a}}$ ) and (b) $\mathrm{H}^{+}$concentration versius $(\mathrm{HA}$ ) Explain both.

Key Concepts

Weak Acid Equilibrium

A weak acid does not completely dissociate in aqueous solution; instead, it establishes a dynamic equilibrium between the undissociated form (HA) and the ions produced (H+ and A–). This equilibrium is governed by the acid dissociation constant (Ka), which has a low value for weak acids, indicating that only a small fraction of the acid molecules lose a proton at any given time.

Percent Dissociation

Percent dissociation is defined as the ratio of the dissociated acid (producing H+ and A–) to the initial concentration of the acid, multiplied by 100. In weak acid solutions, this percentage often decreases as the initial concentration increases because the same equilibrium constant applies, leading to a lower fraction of HA dissociating at higher concentrations due to the mass action effect.

H+ Concentration

The concentration of hydrogen ions (H+) in solution is intrinsically linked to the dissociation of the weak acid. While a higher initial concentration of HA might provide more available acid, the equilibrium established by the dissociation constant means that the relative production of H+ does not increase proportionally; in fact, a higher concentration often results in a smaller fraction of acid molecules dissociating, thus influencing the overall H+ concentration in a non-linear manner.

Equilibrium Constant Expression

The acid dissociation equilibrium can be expressed by the relation Ka = [H+][A–] / [HA]. This expression shows how the concentrations of H+, A–, and the undissociated acid are interdependent. For a weak acid, with a given Ka, changes in the initial concentration of HA affect the relative amounts of its dissociated ions, thereby impacting both the percent dissociation and the hydrogen ion concentration in solution.

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