A 30.0-mL sample of 0.165 M propanoic acid is titrated with 0.300 M KOH. Calculate the pH at each volume of added base: 0 mL.

검증된 단계별 안내

Identify the initial conditions: You have a 30.0 mL sample of 0.165 M propanoic acid (CH₃CH₂COOH) and you are adding 0 mL of 0.300 M KOH.

Recognize that at 0 mL of added base, no KOH has been added, so the solution contains only the weak acid, propanoic acid.

Use the expression for the dissociation of propanoic acid: CH₃CH₂COOH ⇌ CH₃CH₂COO⁻ + H⁺.

Set up the equilibrium expression for the acid dissociation constant (Ka) of propanoic acid: Ka = [CH₃CH₂COO⁻][H⁺] / [CH₃CH₂COOH].

Assume that the initial concentration of H⁺ is negligible and solve for [H⁺] using the initial concentration of propanoic acid and its Ka value to find the pH.

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주요 개념

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Acid-Base Titration

An acid-base titration is a quantitative analytical method used to determine the concentration of an acid or base in a solution. In this process, a solution of known concentration (the titrant) is added to a solution of unknown concentration until the reaction reaches its equivalence point, where the amount of acid equals the amount of base. The pH changes during the titration can be monitored to determine the endpoint.

Weak Acid and Strong Base Reaction

In this scenario, propanoic acid is a weak acid, and KOH is a strong base. When a weak acid reacts with a strong base, the resulting solution will not reach a neutral pH of 7 at the equivalence point. Instead, the pH will be higher than 7 due to the presence of the conjugate base formed from the weak acid, which can hydrolyze in water, affecting the pH.

pH Calculation

The pH of a solution is a measure of its acidity or basicity, calculated using the formula pH = -log[H+]. In a titration, the pH can be determined at various points by considering the concentrations of the acid and base present. For the initial point (0 mL of KOH added), the pH can be calculated using the concentration of the weak acid alone, applying the dissociation constant (Ka) to find the concentration of hydrogen ions.