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Ch.16 - Acid-Base Equilibria
All textbooksBrown 14th EditionCh.16 - Acid-Base EquilibriaProblem 73a
Chapter 16, Problem 73a

Ephedrine, a central nervous system stimulant, is used in nasal sprays as a decongestant. This compound is a weak organic base: C10H15ON1aq2 + H2O1l2 Δ C10H15ONH+1aq2 + OH-1aq2 A 0.035 M solution of ephedrine has a pH of 11.33. (a) What are the equilibrium concentrations of C10H15ON, C10H15ONH+, and OH-?

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We are given a weak base, ephedrine, with a concentration of 0.035 M and a pH of 11.33. We need to find the equilibrium concentrations of C_{10}H_{15}ON, C_{10}H_{15}ONH^+, and OH^-.
Use the pH to find the pOH: \( \text{pOH} = 14 - \text{pH} \). Then, calculate the concentration of OH^- using \( [\text{OH}^-] = 10^{-\text{pOH}} \).
Write the equilibrium expression for the dissociation of the weak base: \( C_{10}H_{15}ON + H_2O \rightleftharpoons C_{10}H_{15}ONH^+ + OH^- \).
Set up an ICE (Initial, Change, Equilibrium) table to express the changes in concentrations. Initially, [C_{10}H_{15}ON] = 0.035 M, and [C_{10}H_{15}ONH^+] and [OH^-] are 0. At equilibrium, [C_{10}H_{15}ON] = 0.035 - x, [C_{10}H_{15}ONH^+] = x, and [OH^-] = x.
Use the equilibrium expression and the calculated [OH^-] to solve for x, which represents the change in concentration. This will give you the equilibrium concentrations of C_{10}H_{15}ON, C_{10}H_{15}ONH^+, and OH^-.

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Key Concepts

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

Acid-Base Equilibrium

Acid-base equilibrium involves the balance between acids and bases in a solution, characterized by the transfer of protons (H+ ions). In this context, ephedrine acts as a weak base, accepting protons from water to form its conjugate acid, C10H15ONH+. Understanding this equilibrium is crucial for calculating the concentrations of the species involved.
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pH and pOH

pH is a measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. In a basic solution, pOH can also be calculated, which is the negative logarithm of the hydroxide ion concentration. The relationship between pH and pOH is given by the equation pH + pOH = 14, which is essential for determining the concentrations of hydroxide ions in this problem.
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Concentration Calculations

Concentration calculations involve determining the molarity of different species in a solution at equilibrium. Given the initial concentration of ephedrine and the pH, one can use the equilibrium expression and the known values to find the concentrations of C10H15ON, C10H15ONH+, and OH-. This requires an understanding of stoichiometry and the principles of chemical equilibrium.
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Related Practice
Textbook Question

Write the chemical equation and the Kb expression for the reaction of each of the following bases with water: (c) benzoate ion, C6H5CO2-

Textbook Question

Ephedrine, a central nervous system stimulant, is used in nasal sprays as a decongestant. This compound is a weak organic base: C10H15ON1aq2 + H2O1l2 Δ C10H15ONH+1aq2 + OH-1aq2 A 0.035 M solution of ephedrine has a pH of 11.33. (b) Calculate Kb for ephedrine.

Textbook Question

Codeine 1C18H21NO32 is a weak organic base. A 5.0 * 10-3M solution of codeine has a pH of 9.95. Calculate the value of Kb for this substance. What is the pKb for this base?