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H₃O⁺

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

Definition

H₃O⁺, also known as the hydronium ion, is a polyatomic ion with the chemical formula H₃O⁺. It is the conjugate acid of water and is a key concept in the understanding of Brønsted-Lowry acid-base theory.

H₃O⁺ cheat sheet for homework

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5 Must Know Facts For Your Next Test

  1. The H₃O⁺ ion is formed when a proton (H⁺) is added to a water molecule (H₂O).
  2. H₃O⁺ is the conjugate acid of water, with water being the conjugate base of H₃O⁺.
  3. The presence of H₃O⁺ ions in a solution indicates that the solution is acidic, with a pH less than 7.
  4. H₃O⁺ is a key participant in many acid-base reactions, acting as a proton donor.
  5. The concentration of H₃O⁺ ions in a solution determines its acidity and is used to calculate the pH of the solution.

Review Questions

  • Explain the role of H₃O⁺ in the Brønsted-Lowry acid-base theory.
    • In the Brønsted-Lowry acid-base theory, H₃O⁺ is considered a Brønsted-Lowry acid because it can donate a proton (H⁺) to another substance, forming a conjugate base. The formation of H₃O⁺ is a key step in many acid-base reactions, as it allows for the transfer of protons between acids and bases. The concentration of H₃O⁺ ions in a solution determines the solution's acidity and is used to calculate the pH, which is a measure of the solution's hydrogen ion concentration.
  • Describe the relationship between H₃O⁺ and the pH of a solution.
    • The concentration of H₃O⁺ ions in a solution is directly related to the pH of that solution. The pH scale is a logarithmic scale that measures the acidity or basicity of a solution, with a pH less than 7 indicating an acidic solution and a pH greater than 7 indicating a basic solution. The pH is calculated as the negative logarithm of the H₃O⁺ ion concentration, so a higher concentration of H₃O⁺ ions corresponds to a lower pH value, and a lower concentration of H₃O⁺ ions corresponds to a higher pH value. This relationship is crucial for understanding the acidity and basicity of solutions in the context of Brønsted-Lowry acid-base theory.
  • Analyze the role of H₃O⁺ in the formation of conjugate acid-base pairs.
    • $$H₃O⁺ + H₂O \leftrightarrow 2H₂O$$ In this equation, H₃O⁺ is the conjugate acid of water (H₂O), and water is the conjugate base of H₃O⁺. This represents a conjugate acid-base pair, where the acid (H₃O⁺) and base (H₂O) can reversibly donate and accept protons. The formation of H₃O⁺ is a key step in many acid-base reactions, as it allows for the transfer of protons between acids and bases. Understanding the role of H₃O⁺ in the formation of conjugate acid-base pairs is essential for predicting and analyzing the behavior of acids and bases in the context of the Brønsted-Lowry theory.
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