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🥼organic chemistry review

H3O+

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

Definition

H3O+ is the hydronium ion, a positively charged species formed when a proton (H+) is added to a water molecule (H2O). It is a key concept in the Brønsted-Lowry definition of acids and bases, where it serves as the conjugate acid of water.

5 Must Know Facts For Your Next Test

  1. The hydronium ion (H3O+) is the conjugate acid of water (H2O) in the Brønsted-Lowry acid-base theory.
  2. H3O+ is formed when a proton (H+) is added to a water molecule, and it is a stronger acid than the original water molecule.
  3. The presence of H3O+ in aqueous solutions is a key indicator of acidic conditions, as it can donate protons to other substances.
  4. The concentration of H3O+ is used to determine the pH of a solution, with lower pH values corresponding to higher H3O+ concentrations.
  5. H3O+ plays a crucial role in many chemical and biological processes, including the dissociation of acids, the regulation of pH, and the transport of protons across cell membranes.

Review Questions

  • Explain the role of H3O+ in the Brønsted-Lowry definition of acids and bases.
    • According to the Brønsted-Lowry theory, an acid is a substance that can donate a proton (H+) to another substance, forming the hydronium ion (H3O+). The hydronium ion is the conjugate acid of water, and it is a stronger acid than the original water molecule. This means that when an acid donates a proton to water, it forms H3O+, which can then go on to donate the proton to another base, continuing the acid-base reaction.
  • Describe the relationship between the concentration of H3O+ and the pH of a solution.
    • The concentration of H3O+ in a solution is directly related to the pH of that solution. The pH scale is a measure of the acidity or basicity of a solution, with lower pH values corresponding to higher H3O+ concentrations. Specifically, the pH is defined as the negative logarithm of the H3O+ concentration, or pH = -log[H3O+]. This means that as the H3O+ concentration increases, the pH decreases, indicating a more acidic solution. Conversely, as the H3O+ concentration decreases, the pH increases, indicating a more basic solution.
  • Analyze the role of H3O+ in the autoionization of water and its significance in acid-base chemistry.
    • The autoionization of water is a process where water molecules can self-ionize to form a small but significant concentration of hydronium (H3O+) and hydroxide (OH-) ions. This is a crucial concept in acid-base chemistry, as the presence of H3O+ and OH- ions determines the pH of the solution. The autoionization of water establishes an equilibrium between these two species, and the relative concentrations of H3O+ and OH- ions are used to define the pH scale. Understanding the role of H3O+ in the autoionization of water and its impact on pH is essential for predicting and analyzing the behavior of acids and bases in various chemical systems and biological processes.

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