Introduction to pH, pOH, and pKw | Summary and Q&A

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September 7, 2009
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Khan Academy
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Introduction to pH, pOH, and pKw

TL;DR

Water spontaneously ionizes to form hydrogen ions (H+) and hydroxide ions (OH-), leading to the concept of pH.

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Questions & Answers

Q: What is the autoionization of water?

Autoionization of water refers to the spontaneous process in which water molecules break apart to form hydronium (H3O+) and hydroxide (OH-) ions in equilibrium.

Q: How are hydrogen bonds involved in water autoionization?

Hydrogen bonds between the oxygen atom of one water molecule and the hydrogen atom of another facilitate the transfer of a hydrogen ion, leading to the formation of hydronium and hydroxide ions.

Q: Why is the concentration of hydronium and hydroxide ions in pure water at 10^-7 M?

The autoionization of water results in a small fraction of water molecules breaking apart into ions. At equilibrium, the concentration of each ion is approximately 10^-7 M in pure water.

Q: What does the pH measure?

pH is a measure of the hydrogen ion concentration in a solution. It indicates the acidity or alkalinity of a substance.

Q: How is pH calculated?

pH is calculated by taking the negative logarithm (base 10) of the hydrogen ion concentration. For example, if the hydrogen ion concentration is 10^-3 M, the pH is equal to 3.

Summary & Key Takeaways

  • Water molecules consist of two hydrogen atoms and one oxygen atom, forming a hydrogen bond that holds them together.

  • Under the right circumstances, water molecules can ionize and form hydronium ions (H3O+) and hydroxide ions (OH-).

  • The concentration of hydronium ions and hydroxide ions in pure water at room temperature is 10^-7 M.

  • The autoionization of water can be described by the equilibrium reaction: H2O ⇄ H3O+ + OH-.

  • The equilibrium constant for this reaction is 10^-14, known as the pKw.

  • pH is a measure of hydrogen ion concentration in a solution. Lower pH values indicate higher acidity, while higher values indicate alkalinity.

  • pH is calculated by taking the negative logarithm (base 10) of the hydrogen ion concentration.

  • The pH of pure water at room temperature is 7, indicating neutrality.

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