Calculate the pH and the concentrations of all species present (H2C8H4O4, HC8H4O4-, C8H4O4 2-, H3O+, and OH-) in a 0.0250 M solution of phthalic acid, H2C8H4O4, with pKa1 = 2.89 and pKa2 = 5.512.

Name: Chemistry
Author: McMurry
ISBN: 9781292336145

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Identify the relevant equilibria for the diprotic acid, H2C8H4O4, which can dissociate in two steps: H2C8H4O4 ⇌ HC8H4O4^- + H^+ and HC8H4O4^- ⇌ C8H4O4^2- + H^+.

Use the first dissociation constant, Ka1, to set up the equilibrium expression: Ka1 = [HC8H4O4^-][H^+]/[H2C8H4O4]. Substitute the initial concentration of H2C8H4O4 and the changes in concentration at equilibrium to solve for [H^+].

Calculate the pH from the [H^+] obtained from the first dissociation using the formula: pH = -log[H^+].

Use the second dissociation constant, Ka2, to set up the equilibrium expression: Ka2 = [C8H4O4^2-][H^+]/[HC8H4O4^-]. Use the concentrations from the first equilibrium to solve for [C8H4O4^2-].

Calculate the concentration of OH^- using the relation Kw = [H^+][OH^-], where Kw is the ion product of water (1.0 x 10^-14 at 25°C).

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Acid-Base Equilibria

Acid-base equilibria involve the dissociation of acids and bases in solution, which can be described using the equilibrium constant expressions. For weak acids like phthalic acid, the dissociation occurs in two steps, each characterized by a specific pKa value. Understanding these equilibria is essential for calculating the concentrations of the various species present in solution.

pH Calculation

pH is a measure of the hydrogen ion concentration in a solution, defined as pH = -log[H3O+]. In the context of weak acids, the pH can be calculated using the concentrations of the acid and its dissociated species, applying the Henderson-Hasselbalch equation when necessary. This calculation is crucial for determining the acidity of the solution and the extent of dissociation of the acid.

Buffer Solutions

Buffer solutions resist changes in pH upon the addition of small amounts of acids or bases. In the case of phthalic acid, the presence of both the weak acid and its conjugate base allows the solution to maintain a relatively stable pH. Understanding how buffers work is important for predicting the behavior of the solution when calculating the concentrations of all species present.