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💏intro to chemistry review

HPO4^2-

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

HPO4^2- is the dihydrogen phosphate ion, a polyatomic ion with a charge of -2. It is an important component in the context of polyprotic acids, which are acids that can donate more than one proton when dissociating in water.

5 Must Know Facts For Your Next Test

  1. The HPO4^2- ion is the dihydrogen phosphate ion, formed as a conjugate base when phosphoric acid (H3PO4) loses two protons.
  2. HPO4^2- is one of the three major conjugate base species of phosphoric acid, along with H2PO4- and PO4^3-.
  3. The presence of HPO4^2- in a solution indicates that the solution is slightly basic, as it is a conjugate base of a polyprotic acid.
  4. The acid dissociation constant (Ka2) for the second dissociation of phosphoric acid to form HPO4^2- is approximately 6.2 x 10^-8.
  5. HPO4^2- is an important buffer species in biological systems, helping to maintain a stable pH in the body.

Review Questions

  • Explain the relationship between HPO4^2- and phosphoric acid (H3PO4) in the context of polyprotic acids.
    • HPO4^2- is the dihydrogen phosphate ion, which is one of the conjugate base species formed when the polyprotic acid phosphoric acid (H3PO4) dissociates in water. Specifically, HPO4^2- is formed when phosphoric acid loses two of its three protons, with the first and second dissociation steps being characterized by the acid dissociation constants Ka1 and Ka2, respectively. The presence of HPO4^2- in a solution indicates that the solution is slightly basic, as it is a conjugate base of a polyprotic acid.
  • Describe the role of HPO4^2- as a buffer species in biological systems.
    • HPO4^2- is an important buffer species in biological systems, such as the human body, that helps maintain a stable pH. As a conjugate base of the polyprotic acid phosphoric acid (H3PO4), HPO4^2- can accept protons to neutralize excess acid, or donate protons to neutralize excess base, thereby resisting changes in pH. This buffering capacity is crucial for the proper functioning of various physiological processes that require a narrow pH range to operate effectively.
  • Analyze the significance of the acid dissociation constant (Ka2) for the formation of HPO4^2- from phosphoric acid.
    • The acid dissociation constant (Ka2) for the second dissociation of phosphoric acid (H3PO4) to form HPO4^2- is an important parameter that reflects the strength of this acid-base equilibrium. A lower Ka2 value, such as the approximately 6.2 x 10^-8 for phosphoric acid, indicates that the formation of HPO4^2- is less favored compared to the first dissociation step. This means that a larger proportion of the phosphoric acid will exist as the H2PO4- conjugate base species under typical conditions. The Ka2 value, therefore, provides insight into the relative abundance of the HPO4^2- ion and its role in the overall acid-base behavior of polyprotic systems.