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Learning objective

Use Kw to calculate hydrogen ion or hydroxide ion concentrations.

Read the explanation, check the common trap, then practise with flashcards and questions.

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Topic

Acids and bases (A-level only)

Subtopic

pH and Kw (A-level only)

Aqa A Level ChemistryPhysical chemistry

Study support

Understand this objective

Quick explanation

Use Kw to calculate hydrogen ion or hydroxide ion concentrations

  • This point belongs to Acids and bases (A-level only), especially pH and Kw (A-level only).
  • You need to be able to use Kw to calculate hydrogen ion or hydroxide ion concentrations.
  • The key ideas to know are hydroxide ion and Kw.
  • Use the linked flashcards and practice questions to check recall, then practise applying the idea in an exam-style answer.

Key concepts

hydroxide ionKw

Why it matters

This objective helps connect pH and Kw (A-level only) to exam-style questions, flashcards, and revision notes for Acids and bases (A-level only).

Quick student answer

What is the value of Kw at 25°C?

Direct answer

1.0 x 10^-14

Key terms

  • pH: A measure of the acidity or basicity of a solution, calculated as the negative logarithm of the hydrogen ion concentration.
  • Kw: The ion product of water, equal to 1.0 x 10^-14 at 25°C, representing the equilibrium constant for the self-ionization of water.

Common trap

Incorrectly calculating [OH⁻] from pH: Always use [H⁺] = 10^-pH to find the hydrogen ion concentration before calculating [OH⁻].

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