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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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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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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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