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Ch.18 - Aqueous Ionic Equilibrium
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.18 - Aqueous Ionic EquilibriumProblem 79e
Chapter 18, Problem 79e

Consider the titration of a 25.0-mL sample of 0.175 M CH3NH2 with 0.150 M HBr. Determine each quantity. e. the pH at the equivalence point

Verified step by step guidance
1
Identify the reaction: CH_3NH_2 (aq) + HBr (aq) \(\rightarrow\) CH_3NH_3^+ (aq) + Br^- (aq).
Determine the moles of CH_3NH_2 initially present using the formula: \(\text{moles}\) = \(\text{concentration}\) \(\times\) \(\text{volume}\).
Calculate the volume of HBr needed to reach the equivalence point using the stoichiometry of the reaction and the concentrations given.
At the equivalence point, all CH_3NH_2 is converted to CH_3NH_3^+. Determine the concentration of CH_3NH_3^+ in the solution.
Use the K_b of CH_3NH_2 to find the K_a of CH_3NH_3^+ and calculate the pH using the formula: \(\text{pH}\) = -\(\log\)[H^+].

Key Concepts

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

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant (a solution of known concentration) to a sample until the reaction reaches its equivalence point, where the amount of titrant equals the amount of substance in the sample. Understanding the stoichiometry of the reaction is crucial for calculating concentrations and pH at various points during the titration.
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Acid-Base Titration

Equivalence Point

The equivalence point in a titration is the stage at which the amount of titrant added is stoichiometrically equivalent to the amount of substance present in the sample. At this point, the reaction between the acid and base is complete, and the pH of the solution can change dramatically. For a weak base like methylamine (CH3NH2) being titrated with a strong acid like HBr, the pH at the equivalence point will be determined by the properties of the resulting salt and the hydrolysis of its ions.
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At the Equivalence Point

pH Calculation

The pH of a solution is a measure of its acidity or basicity, calculated as the negative logarithm of the hydrogen ion concentration. At the equivalence point of a titration involving a weak base and a strong acid, the resulting solution will contain the conjugate acid of the weak base, which can affect the pH. To find the pH at this point, one must consider the concentration of the conjugate acid and its dissociation in water, often requiring the use of the acid dissociation constant (Ka) for accurate calculations.
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pH Calculation Example
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