Asked by ny
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.
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Answered by
GPT-5 mini
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Statement 3 is best.
Pure water undergoes self‑ionization (autoprotolysis): 2 H2O ⇌ H3O+ + OH−. At 25 °C the ion‑product Kw = [H3O+][OH−] = 1.0×10−14, so in pure water [H3O+] = [OH−] = √Kw = 1.0×10−7 M. pH = −log[H3O+] = 7. (pH 7 is temperature dependent because Kw changes with temperature.)
Pure water undergoes self‑ionization (autoprotolysis): 2 H2O ⇌ H3O+ + OH−. At 25 °C the ion‑product Kw = [H3O+][OH−] = 1.0×10−14, so in pure water [H3O+] = [OH−] = √Kw = 1.0×10−7 M. pH = −log[H3O+] = 7. (pH 7 is temperature dependent because Kw changes with temperature.)
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