A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: 40 mL.

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Identify the reaction: Pyridine (C5H5N) is a weak base and reacts with HCl, a strong acid, to form pyridinium ion (C5H5NH+).

Calculate the initial moles of pyridine using its concentration and volume: \( \text{moles of pyridine} = 0.125 \text{ M} \times 0.025 \text{ L} \).

Calculate the moles of HCl added using its concentration and volume: \( \text{moles of HCl} = 0.100 \text{ M} \times 0.040 \text{ L} \).

Determine the limiting reactant and calculate the moles of pyridinium ion formed and any excess reactant remaining after the reaction.

Use the Henderson-Hasselbalch equation to calculate the pH: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right) \), where \( \text{pK}_a \) is derived from the \( K_b \) of pyridine.

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Key Concepts

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

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, and monitoring these changes allows for the calculation of the unknown concentration.

Buffer Solutions

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. In the context of the titration of pyridine (a weak base) with HCl (a strong acid), the buffer capacity is crucial for understanding how the pH will change before and after the equivalence point.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a buffer solution. It relates the pH of the solution to the pKa of the weak acid and the ratio of the concentrations of the conjugate base and the weak acid. This equation is particularly useful in titration problems, as it allows for the determination of pH at various points during the titration, especially when the solution is still in the buffer region.

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Henderson-Hasselbalch Equation

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Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: 20 mL.

Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: equivalence point.

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Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: one-half equivalence.

Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: 50 mL.

Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: 0 mL, 10 mL, 20 mL, equivalence point, one-half equivalence point, 40 mL, 50 mL. Sketch the titration curve.

Textbook Question

Consider the titration curves (labeled a and b) for two weak acids, both titrated with 0.100 M NaOH.

(i) Which acid solution is more concentrated?