Calculate the pH at the equivalence point for titrating 0.200 M solutions of each of the following bases with 0.200 M HBr: (a) sodium hydroxide (NaOH).

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Identify the type of titration: This is a strong base (NaOH) being titrated with a strong acid (HBr). At the equivalence point, the moles of NaOH will equal the moles of HBr added.

Write the balanced chemical equation for the reaction: NaOH + HBr → NaBr + H₂O. This shows that NaOH reacts with HBr to form NaBr and water.

Determine the concentration of the resulting solution at the equivalence point: Since both solutions are 0.200 M and mixed in equal volumes, the resulting solution will be neutral, as NaBr is a neutral salt.

Understand the pH of a neutral solution: For a strong acid-strong base titration, the pH at the equivalence point is determined by the nature of the salt formed. NaBr does not hydrolyze in water, so the solution remains neutral.

Conclude the pH at the equivalence point: Since the solution is neutral and neither the cation (Na⁺) nor the anion (Br⁻) affects the pH, the pH at the equivalence point is 7.

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Key Concepts

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

Titration and Equivalence Point

Titration is a quantitative analytical method used to determine the concentration of a solute in a solution. The equivalence point occurs when the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample. At this point, the reaction between the acid and base is complete, and the pH of the solution is determined by the properties of the resulting salt and any remaining species.

pH and its Calculation

pH is a measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. At the equivalence point of a strong base like sodium hydroxide (NaOH) titrated with a strong acid like HBr, the resulting solution contains the salt (NaBr) and water. The pH can be calculated based on the hydrolysis of the salt, which may affect the concentration of hydrogen ions in the solution.

Hydrolysis of Salts

When a salt dissolves in water, it can undergo hydrolysis, where the ions react with water to form either acidic or basic solutions. In the case of NaBr, the sodium ion does not hydrolyze, while the bromide ion is the conjugate base of a strong acid (HBr) and does not affect the pH significantly. Therefore, the pH at the equivalence point for the titration of NaOH with HBr will be neutral, around 7, since neither ion contributes to acidity or basicity.

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