Textbook Question
Write chemical equations and corresponding equilibrium expressions for each of the three ionization steps of phosphoric acid.
17. Acid and Base Equilibrium
Triprotic Acids and Bases
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Determine the equilibrium expression for the Ka3 value of citric acid, H3C6H5O7?
A
Ka3 = [H2C6H5O7−][H3O+]/[H3C6H5O7]
B
Ka3 = [HC6H5O72−][H3O+]/[H2C6H5O7−]
C
Ka3 = [C6H5O73−][H3O+]/[HC6H5O72−]
D
Ka3 = [C6H5O73−][H3O+]/[HC6H5O72−][H2O]
Verified step by step guidance1
Understand that citric acid, H3C6H5O7, is a triprotic acid, meaning it can donate three protons (H+ ions) in solution. Each dissociation step has its own equilibrium constant, Ka.
Identify the third dissociation step of citric acid, which involves the conversion of HC6H5O7^2− to C6H5O7^3− and the release of an H+ ion. This is represented by the equation: HC6H5O7^2− ⇌ C6H5O7^3− + H+.
Recognize that the equilibrium expression for this third dissociation step, Ka3, is based on the concentrations of the products and reactants at equilibrium. The expression is: Ka3 = [C6H5O7^3−][H3O+]/[HC6H5O7^2−].
Note that water (H2O) is typically not included in the equilibrium expression for acids and bases because it is the solvent and its concentration is considered constant. Therefore, the correct equilibrium expression does not include [H2O].
Verify that the correct equilibrium expression for the third dissociation step of citric acid is: Ka3 = [C6H5O7^3−][H3O+]/[HC6H5O7^2−], which matches the given correct answer.
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