Calculate the molar concentration of OH- in a 0.075 M solution of ethylamine (C2H5NH2); Kb = 6.4 * 10^-4. Calculate the pH of this solution.

Name: Chemistry: The Central Science
Author: Brown
ISBN: 9780134414232

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Identify the reaction: Ethylamine (C2H5NH2) is a weak base and will react with water to form C2H5NH3+ and OH-. Write the equilibrium expression for this reaction.

Write the expression for the base dissociation constant (Kb): Kb = [C2H5NH3+][OH-] / [C2H5NH2].

Set up an ICE (Initial, Change, Equilibrium) table to determine the concentrations of the species at equilibrium. Assume the initial concentration of OH- and C2H5NH3+ is 0, and the change in concentration for C2H5NH2 is -x, for OH- and C2H5NH3+ is +x.

Substitute the equilibrium concentrations from the ICE table into the Kb expression: Kb = (x)(x) / (0.075 - x).

Solve for x, which represents the concentration of OH- at equilibrium. Use the approximation method if x is small compared to the initial concentration, then calculate the pH using the relation pH = 14 - pOH, where pOH = -log[OH-].

주요 개념

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Base Ionization and Kb

Ethylamine (C2H5NH2) is a weak base that partially ionizes in water to produce hydroxide ions (OH-) and its conjugate acid (C2H5NH3+). The base dissociation constant (Kb) quantifies the extent of this ionization, with a higher Kb indicating a stronger base. In this case, Kb = 6.4 x 10^-4 helps determine the concentration of OH- produced in the solution.

Equilibrium Concentrations

To find the concentration of OH- in the solution, we set up an equilibrium expression based on the ionization of ethylamine. Using an ICE (Initial, Change, Equilibrium) table, we can express the changes in concentrations of the reactants and products at equilibrium. This allows us to solve for the concentration of OH- using the Kb value.

pH and pOH Calculations

The pH of a solution is a measure of its acidity, while pOH measures its basicity. They are related through the equation pH + pOH = 14. Once the concentration of OH- is determined, the pOH can be calculated using the formula pOH = -log[OH-]. Subsequently, the pH can be found by rearranging the equation to pH = 14 - pOH.