Find the pH and percent ionization of a 0.100 M solution of a weak monoprotic acid given the following Ka values: a. Ka = 1.0 * 10^-5, b. Ka = 1.0 * 10^-3, c. Ka = 1.0 * 10^-1.

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Identify the expression for the ionization of a weak monoprotic acid: HA ⇌ H⁺ + A⁻.

Write the expression for the acid dissociation constant (Ka): Ka = [H⁺][A⁻]/[HA].

Assume the initial concentration of HA is 0.100 M and the change in concentration due to ionization is x, so [H⁺] = [A⁻] = x and [HA] = 0.100 - x.

Substitute these values into the Ka expression: Ka = x^2 / (0.100 - x).

Solve for x (which represents [H⁺]) using the quadratic formula or by assuming x is small compared to 0.100 M, then calculate pH = -log[H⁺] and percent ionization = (x/0.100) * 100%.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Weak Acid Ionization

Weak acids do not completely dissociate in solution, which means only a fraction of the acid molecules release protons (H+). The extent of this ionization is quantified by the acid dissociation constant (Ka), which indicates the strength of the acid. A lower Ka value corresponds to a weaker acid and less ionization, while a higher Ka indicates a stronger acid with greater ionization.

pH Calculation

The pH of a solution is a measure of its acidity, calculated using the formula pH = -log[H+], where [H+] is the concentration of hydrogen ions in the solution. For weak acids, the concentration of H+ can be determined from the equilibrium expression derived from the acid dissociation constant (Ka) and the initial concentration of the acid. Understanding how to set up and solve this equilibrium expression is crucial for calculating pH.

Percent Ionization

Percent ionization is a way to express the degree to which a weak acid dissociates in solution, calculated as (amount of acid ionized / initial concentration of acid) × 100%. This value provides insight into the strength of the acid; a higher percent ionization indicates a stronger acid. It is particularly useful for comparing the ionization of acids with different Ka values in similar concentrations.