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Ch.17 - Aqueous Ionic Equilibrium
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook
All textbooksTro 4th EditionCh.17 - Aqueous Ionic EquilibriumProblem 34
Chapter 17, Problem 34

Solve an equilibrium problem (using an ICE table) to calculate the pH of each solution. a. 0.18 M CH3NH2 b. 0.18 M CH3NH3Cl c. a mixture that is 0.18 M in CH3NH2 and 0.18 M in CH3NH3Cl

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Identify the species in solution and their roles: CH3NH2 is a weak base, and CH3NH3Cl is a salt that can provide CH3NH3+, the conjugate acid of CH3NH2.
For part (a), set up an ICE table for the equilibrium: CH3NH2 + H2O ⇌ CH3NH3+ + OH-. Use the initial concentration of CH3NH2 (0.18 M) and assume initial concentrations of CH3NH3+ and OH- are 0. Define the change in concentration as -x for CH3NH2 and +x for both CH3NH3+ and OH-.
Write the expression for the base dissociation constant (Kb) of CH3NH2: Kb = [CH3NH3+][OH-] / [CH3NH2]. Substitute the equilibrium concentrations from the ICE table into this expression.
For part (b), recognize that CH3NH3Cl dissociates completely in water to give CH3NH3+ and Cl-. Set up an ICE table for the equilibrium: CH3NH3+ + H2O ⇌ CH3NH2 + H3O+. Use the initial concentration of CH3NH3+ (0.18 M) and assume initial concentrations of CH3NH2 and H3O+ are 0. Define the change in concentration as -x for CH3NH3+ and +x for both CH3NH2 and H3O+.
For part (c), recognize that the solution is a buffer containing both CH3NH2 and CH3NH3+. Use the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]). Calculate pKa from the known Kb of CH3NH2 and the relation pKa + pKb = 14. Substitute the concentrations of CH3NH2 and CH3NH3+ into the equation to find the pH.

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Key Concepts

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

Equilibrium and ICE Tables

Equilibrium in chemistry refers to the state where the concentrations of reactants and products remain constant over time. An ICE table (Initial, Change, Equilibrium) is a tool used to organize the concentrations of species involved in a reaction at different stages. It helps in calculating the changes in concentration as the system reaches equilibrium, which is essential for determining pH in acid-base reactions.
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Acid-Base Chemistry

Acid-base chemistry involves the study of substances that can donate protons (acids) or accept protons (bases). In this question, CH3NH2 (methylamine) is a weak base, while CH3NH3Cl (methylammonium chloride) is its conjugate acid. Understanding the strength of these acids and bases, as well as their dissociation in water, is crucial for calculating the pH of the solutions.
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pH Calculation

pH is a measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. For weak acids and bases, the pH can be calculated using the equilibrium concentrations derived from the ICE table. In mixtures of weak acids and their conjugate bases, the Henderson-Hasselbalch equation can also be applied to find the pH, highlighting the relationship between pH, pKa, and the concentrations of the acid and base.
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