Textbook Question
What is the pH and the principal source of H3O+ ions in 1.0 * 10-10 M HCl? (Hint: The pH of an acid solution can’t exceed 7.) What is the pH of 1.0 * 10-7 M HCl?
Ch.16 - Aqueous Equilibria: Acids & Bases
Chapter 16, Problem 17.142
The acidity of lemon juice is derived primarily from citric acid (H3Cit), a triprotic acid. What are the concentrations of H3Cit, H2Cit-, HCit2-, and Cit3- in a sample of lemon juice that has a pH of 2.37 and a total concentration of the four citrate-containing species of 0.350 M?
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Identify the relevant acid dissociation constants (Ka) for citric acid (H3Cit). Citric acid is a triprotic acid, meaning it can donate three protons. The dissociation reactions are: H3Cit
ightarrow H2Cit^- + H^+, H2Cit^-
ightarrow HCit^{2-} + H^+, and HCit^{2-}
ightarrow Cit^{3-} + H^+.
Write the expression for the pH of the solution and relate it to the concentration of H+ ions. Since pH = -log[H+], calculate [H+] using the pH given (2.37).
Set up the equilibrium expressions for each dissociation step using the Ka values and the concentrations of H3Cit, H2Cit-, HCit2-, and Cit3-. Use the formula Ka = [Products]/[Reactants] for each step.
Since the total concentration of all citrate species (H3Cit, H2Cit-, HCit2-, Cit3-) is given as 0.350 M, use this information to set up an equation summing the concentrations of all species to equal 0.350 M.
Solve the system of equations derived from the equilibrium expressions and the total concentration equation to find the individual concentrations of H3Cit, H2Cit-, HCit2-, and Cit3-.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Triprotic Acids
Triprotic acids, like citric acid, can donate three protons (H+) in a stepwise manner. Each deprotonation step has a distinct dissociation constant (Ka), which indicates the strength of the acid at each stage. Understanding the behavior of triprotic acids is crucial for calculating the concentrations of their various ionic forms in solution.
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Triprotic Acid Equilibrium
pH and Concentration Relationship
The pH of a solution is a measure of its hydrogen ion concentration, defined as pH = -log[H+]. In this case, a pH of 2.37 corresponds to a hydrogen ion concentration of approximately 0.0042 M. This relationship is essential for determining the concentrations of the different species of citric acid in lemon juice.
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Equilibrium and Species Concentration
In a solution containing a weak acid and its conjugate bases, the concentrations of the various species are determined by the acid dissociation equilibria. For citric acid, the total concentration of all species must equal the given total concentration (0.350 M), and the individual concentrations can be calculated using the equilibrium constants and the pH value.
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Related Practice
Textbook Question
Calculate the pH of solutions prepared by:
(d) Mixing equal volumes of 0.20 M HCl and 0.50 M HNO3.
(Assume that volumes are additive.)
Textbook Question
A 100.0 mL sample of a solution that is 0.100 M in HCl and 0.100 M in HCN is titrated with 0.100 M NaOH. Calculate the pH after the addition of the following volumes of NaOH:
(a) 0.0 mL
Textbook Question
A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equivalence point was reached after 126.4 mL of base.
(e) Sketch the pH titration curve, and label the buffer regions and equivalence points.
Textbook Question
The following pictures represent aqueous solutions of three acids HA1A = X, Y, or Z2; water molecules have been omitted for clarity.
(c) Which acid, if any, is a strong acid?