Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 46e,f

Chapter 17, Problem 46e,f

Consider the titration of 30.0 mL of 0.050 M NH₃ with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (e) 61.0 mL (f) 65.0 mL.

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Identify the reaction: NH_3 (aq) + HCl (aq) \rightarrow NH_4^+ (aq) + Cl^- (aq).

Calculate the initial moles of NH_3 using its concentration and volume: \text{moles NH}_3 = 0.050 \text{ M} \times 0.030 \text{ L}.

Calculate the moles of HCl added using its concentration and the given volume: \text{moles HCl} = 0.025 \text{ M} \times 0.061 \text{ L}.

Determine the limiting reactant and calculate the moles of NH_3 and NH_4^+ after the reaction.

Use the Henderson-Hasselbalch equation to find the pH: \text{pH} = \text{p}K_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right), where \text{p}K_a is for NH_4^+.

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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 solution of the analyte (the solution of unknown concentration) until the reaction reaches its endpoint, which is often indicated by a color change or a pH change.

Buffer Solutions

Buffer solutions are mixtures that can resist changes in pH upon the addition of small amounts of acid or base. In the context of the titration of ammonia (NH3), which is a weak base, the resulting solution will contain both the weak base and its conjugate acid (NH4+), forming a buffer that helps maintain the pH within a certain range even as HCl is added.

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. In titration problems, pH can be determined by considering the concentrations of the reactants and products at various stages of the titration, particularly at the equivalence point and beyond, where the solution may become acidic or basic depending on the nature of the reactants.

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Consider the titration of 30.0 mL of 0.050 M NH₃ with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (b) 20.0 mL (c) 59.0 mL.

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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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Calculate the pH at the equivalence point in titrating 0.100 M solutions of each of the following with 0.080 M NaOH: (a) hydrobromic acid (HBr).

Consider the titration of 30.0 mL of 0.050 M NH3 with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (e) 61.0 mL (f) 65.0 mL.

Verified video answer for a similar problem:

Key Concepts

Titration

Buffer Solutions

pH Calculation

Consider the titration of 30.0 mL of 0.050 M NH₃ with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (e) 61.0 mL (f) 65.0 mL.