What is the pH at the equivalence point for the titration of 0.10 M solutions of the following acids and bases, and which of the indicators in Figure 17.5 would be suitable for each titration? (c) CH3NH2 (methylamine) and HCl

Verified step by step guidance

1. Identify the nature of the acid and base involved in the titration. In this case, CH3NH2 (methylamine) is a weak base and HCl (hydrochloric acid) is a strong acid.
2. Understand that at the equivalence point, the amount of acid added equals the amount of base originally present. The resulting solution will primarily contain the conjugate acid of the weak base (CH3NH3+ in this case) because the strong acid (HCl) will completely dissociate and neutralize the weak base.
3. Recognize that the pH at the equivalence point is determined by the hydrolysis of the conjugate acid (CH3NH3+). The reaction of CH3NH3+ with water (hydrolysis) produces CH3NH2 and H3O+ (hydronium ions), making the solution slightly acidic.
4. Calculate the pH using the formula for the hydrolysis of the conjugate acid. Use the expression for the equilibrium constant of the hydrolysis reaction, which can be related to the Kb of the base (CH3NH2) and the Kw of water. The formula to use is: pH = 0.5 * (pKa + log [salt]/[acid]), where pKa is the negative log of the acid dissociation constant of the conjugate acid.
5. Choose an appropriate indicator for the titration. The indicator should have a color change range that includes the pH at the equivalence point. For a slightly acidic equivalence point, an indicator like methyl red or bromocresol green, which changes color around pH 5-6, would be suitable.

Key Concepts

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 can be determined based on the nature of the resulting solution.

pH and Acid-Base Properties

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity. At the equivalence point of a titration involving a weak base (like methylamine) and a strong acid (like HCl), the resulting solution contains the conjugate acid of the weak base, which can affect the pH. Understanding the properties of the acid and base involved is crucial for predicting the pH at this point.

Indicators in Titration

Indicators are substances that change color at a specific pH range and are used to signal the endpoint of a titration. The choice of indicator depends on the pH at the equivalence point of the titration. For the titration of methylamine with HCl, an appropriate indicator would be one that changes color around the pH of the equivalence point, which is typically acidic due to the formation of the conjugate acid.