Which of these statements best explains why pure water has a pH of 7?Pure water contains only H2O molecules, but pH is based on the concentration of H3O+ ions.Each molecule of water can dissociate to form 7 H3O+ ions.Water self-ionizes to form an equilibrium system that has [H3O+] = 1.0 × 10–7 M.
Answer
A
Pure water contains only H2O molecules, but pH is based on the concentration of H3O+ ions.
B
Each molecule of water can dissociate to form 7 H3O+ ions.
C
Water self-ionizes to form an equilibrium system that has [H3O+] = 1.0 × 10–7 M.
Explanation
pH is calculated as −log₁₀[H₃O⁺]. In pure water at 25°C, a small fraction of water molecules self-ionize, establishing an equilibrium with [H₃O⁺] = 1.0 × 10⁻⁷ M. Therefore, pH = −log₁₀(1.0 × 10⁻⁷) = 7, so the statement about water self-ionizing to that concentration is correct.
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