Consider the following dissociation reaction of a weak acid,...

Consider the following dissociation reaction of a weak acid, HA: $$ \mathrm{HA}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\ell) \rightleftarrows \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{A}^{-}(\mathrm{aq}) $$ It was determined that $2.63 \%$ of the acid in a 0.150 M solution of HA in water was dissociated. Calculate the pH of the 0.150 M solution.

Consider the following dissociation reaction of a weak acid, HA:

$$
\mathrm{HA}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\ell) \rightleftarrows \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{A}^{-}(\mathrm{aq})
$$

It was determined that $2.63 \%$ of the acid in a 0.150 M solution of HA in water was dissociated. Calculate the pH of the 0.150 M solution.

Key Concepts

Weak Acid

A weak acid is a substance that only partially dissociates into its ions when dissolved in water. Unlike strong acids, which fully ionize, weak acids reach an equilibrium where both the undissociated acid and its dissociated ions coexist. The degree of dissociation plays a crucial role in determining the acid's inherent strength, its ion concentrations at equilibrium, and consequently its effect on the pH of the solution.

Acid-Base Equilibrium

Acid–base equilibrium refers to the state in which the rate of the forward reaction (acid dissociation) equals the rate of the reverse reaction (recombination of ions) in an aqueous solution. This balance determines the concentrations of all species present in the solution at equilibrium and is typically described by an equilibrium constant. Understanding this equilibrium is fundamental when analyzing reactions involving acids and bases, especially for weak acid systems.

Percent Dissociation

Percent dissociation is the fraction of the original acid that ionizes in solution, expressed as a percentage. For a weak acid, this percentage is usually small, indicating that the majority of the acid remains undissociated at equilibrium. Calculating percent dissociation provides important insights into the acid's strength and its contribution to the concentration of hydronium ions, which in turn affects the pH of the solution.

Molarity

Molarity is a measure of concentration, defined as the number of moles of solute per liter of solution. It is a fundamental unit in chemistry that is used to quantify how much of a given substance is present in a specific volume of solution. In acid-base reactions, molarity helps determine the initial amount of acid present, which is essential for computing the concentrations of ions at equilibrium.

pH and pH Calculation

pH is a quantitative measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydronium ion concentration. In acid-base equilibria, calculating the pH involves determining the equilibrium concentration of the hydronium ions generated by the dissociation of the acid and then applying the pH formula. This calculation is critical for predicting the behavior of solutions in various chemical contexts.

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