Acid Equilibrium: Understanding Phosphoric Acid Dissociation
What determines the composition of an aqueous solution of phosphoric acid?
A. An aqueous solution of phosphoric acid contains mainly H3PO4 molecules.
B. An aqueous solution of phosphoric acid contains predominantly H3O+ and H2PO4- ions.
C. An aqueous solution of phosphoric acid contains equal amounts of H3O+ and H3PO4.
D. An aqueous solution of phosphoric acid contains a greater concentration of dissolved ions than it does neutral phosphoric acid molecules.
Answer:
An aqueous solution of phosphoric acid will predominantly contain H3PO4 molecules, with smaller amounts as H3O+ and H2PO4- ions.
When phosphoric acid (H3PO4) dissolves in water, an equilibrium is established. The equilibrium constant Ka for H3PO4 is 7.5 × 10-3, indicating that the dissociation of phosphoric acid is not very favorable at standard conditions.
Due to its low Ka value, the equilibrium lies to the left towards the reactants. This means that an aqueous solution of phosphoric acid will predominantly contain H3PO4 molecules, with smaller amounts of H3O+ and H2PO4- ions.
The composition of the solution is determined by the equilibrium position, which in turn is influenced by the strength of the acid. In this case, the predominance of H3PO4 molecules reflects the nature of the equilibrium in the dissociation of phosphoric acid.
Understanding acid equilibrium, such as in the dissociation of phosphoric acid, is crucial in predicting the behavior of acids in solution and their impact on chemical reactions.